Nope - that did not help. And I think the problem is a bit unrelated. Mkgmap with your new version and --improve-overview assumes there is an empty map - and puts those three square areas of missing anything. Maybe you give it a try and analyse what happens.
I actually think this bug has been there before already - and it happened to me with contourlines only maps. Meaning mkgmap cuts the map area way too aggressively, and removes part of the map that actually even have content - or was it splitter. I think for contourlines only maps I had to use splitter with --keep-complete and --no-trim else there would be missing. areas in the map. (not on the outside but right in the middle in the ocean usually - but sometimes also e.g. northernmost parts of Norway in an Europe contourlines only map).
So I split australia-oceania WITHOUT --no-trim. Are you assuming to split with --no-trim? Actually those grey boxes are each of its own separate tiles (with nearly no content - e.g. one has a riff polygon only). I guess there is lots of openseamap stuff so splitter creates those tiny tiles...
Mkgmap should never trip on the overview map. The normal map should be trimmed aggressively - as we still cannot cut according to a bounding polygon and only using rectangles. But the overview map sea creation should not cut out sea or anything.
I guess you never noticed that working on Norway - but it would be a good start to play around with australia-oceania (it is nice to work with - because it is relatively small in size so compiles fast). See attached screenshot of overview map. All the selected tiles were before showing sea (and do so once past the overview map too). The high DP filter was not the problem. For Australia-Oceania the --improve-overview right now is not a good example. The map was better before. Oh yeah - and --polygon-size-limits="24:12, 23:14, 22:14, 21:20, 20:20, 19:20, 18:20, 17:20, 16:20, 15:20, 14:20, 13:20, 12:20, 11:20, 10:20
looks better than small values. Actually I think even to rather increase more. Maybe that is why I had less problems right from the start - because I cut away tiny islands.
C:\openmtbmap\maps>if yes NEQ no start /low /b /wait java -jar -XX:+AggressiveHeap -XX:StringTableSize=1000003 -Xms15000M -Xmx29000M C:\openmtbmap\mkgmap.jar --max-jobs=8 --order-by-decreasing-area "--generate-sea" --code-page=1252 "--precomp-sea=C:\openmtbmap\maps\sea.zip" "--style-file=C:\openmtbmap\openmtbmap_style" --improve-overview --add-boundary-nodes-at-admin-boundaries --drive-on=detect --allow-reverse-merge --line-types-with-direction=0x18,0x1f,0x10016,0x10101,0x10105,0x10401,0x10403,0x10405,0x10408,0x10409,0x1040b,0x1040d,0x1040e,0x10500,0x10501,0x10502,0x10503,0x10504,0x10505,0x10506,0x10507,0x10607,0x10609,0x1060b,0x10702,0x10703,0x10704,0x10705,0x10706,0x10707,0x10a00,0x10a02,0x10a04,0x10a05,0x10e1e,0x10e0f,0x10e0e,0x10e1e,0x10f0a,0x10f1c,0x10e1d --nsis --index --levels="0:24, 1:23, 2:22, 3:21, 4:20, 5:19, 6:18" --overview-levels="7:17, 8:16, 9:15, 10:14, 11:13, 12:12" --add-pois-to-areas --pois-to-areas-placement=entrance=main;entrance=yes;building=entrance;barrier=entrance --x-simplify-filter-line-errors=23:2.6,22:4.2,21:5.4,20:6,19:7,18:7.5,17:6,14:6 --x-simplify-filter-polygon-errors=23:3.6,22:7,21:6,20:9,17:2.6 --add-boundary-nodes-at-admin-boundaries=2 --poi-excl-index=0x6405,0x4316,0x2f00 --copyright-file="C:\openmtbmap\openmtbmap_svn\copyrightopm.txt" --license-file="C:\openmtbmap\openmtbmap_svn\licenseopm.txt" --cycle-map --ignore-fixme-values --housenumbers --road-name-config=C:\openmtbmap\openmtbmap_svn\roadNameConfig.txt --split-name-index --link-pois-to-ways --ignore-turn-restrictions --polygon-size-limits="24:12, 23:14, 22:14, 21:20, 20:20, 19:20, 18:20, 17:20, 16:20, 15:20, 14:20, 13:20, 12:20, 11:20, 10:20" --description=omtb_auo --show-profiles=1 --location-autofill=bounds,is_in,nearest --bounds=C:\openmtbmap\maps\bounds.zip --mdr7-del=pri;sec;ter;cw;min;unsf,uncl;living;pdstrn;trk;pth;ft;fp;brdlw;rd;byw;ser;cw;mrt;crt;Bklane;Bktrk;opp;Opptrk;g0;g1;g2;g3;g4;g5;g6;t1;t2;t3;t4;t5;t6;xbk;cr;mr;hr;mr0;mr1;mr2;mr3;mr4;mr5;mr0.;mr1.;mr2.;mr3.;mr4.;mr5.;mr6.;mr00;mr01;mr02;mr03;mr04;mr05;mr06;mr11;mr12;mr13;mr14;mr15;mr16;mr20;mr21;mr22;mr23;mr24;mr25;mr26;mr30;mr31;mr32;mr33;mr34;mr35;mr35;mr40;mr41;mr41;mr43;mr44;mr45;mr46;mr50;mr51;mr52;mr53;mr54;mr55;mr56;mr60;mr61;mr62;mr63;mr64;mr65;mr66;m0;m1;m2;m3;m4;m5;m0.;m1.;m2.;m3.;m4.;m5.;m6.;m00;m01;m02;m03;m04;m05;m06;m11;m12;m13;m14;m15;m16;m20;m21;m22;m23;m24;m25;m26;m30;m31;m32;m33;m34;m35;m35;m40;m41;m41;m43;m44;m45;m46;m50;m51;m52;m53;m54;m55;m56;m60;m61;m62;m63;m64;m65;m66;mr.0;mr.1;mr.2;mr.3;mr.4;mr.5;mr.6;m.0;m.1;m.2;m.3;m.4;m.5;m.6;m1.;m2.;m3.;m4.;m5.;m6.;lmr;mrx0;mrx1;mrx2;mrx3;mrx4;mrx5;mrx6;mx0;mx1;mx2;mx3;mx4;mx5;mx6;bridge;swing;aqueduct;viaduct;"ntnl bndry";admin_lv=4;"ntnl park";weir;dam;waterfall;rapids;stream;river;drain;ditch;"huge river";"hugest river";"medium river";"small river";"wide river";"hugest canal";"huge canal";"medium canal";"small canal";"big canal";canal;snowpark;Skitour-descent;Skipiste;Skislope;illuminated;Nordic-Skiing;skitour;platter;ropetow;tbar;chairlift;goods_lift;runway;taxiway;apron;VFS0;VFS1;VFS2;VFS3;VFS4;VFS5;VFS6;VFS7;usf;cstrn;steps;parking_aisle;res;cob;cobstn;public_footway;public_footpath --mdr7-excl=pri;sec;ter;cw;min;unsf,uncl;pdstrn;trk;pth;ft;fp;brdlw;rd;byw;ser;cw;mrt;crt;Bklane;Bktrk;opp;Opptrk;g0;g1;g2;g3;g4;g5;g6;t1;t2;t3;t4;t5;t6;xbk;cr;mr;hr;mr0;mr1;mr2;mr3;mr4;mr5;mr0.;mr1.;mr2.;mr3.;mr4.;mr5.;mr6.;mr00;mr01;mr02;mr03;mr04;mr05;mr06;mr11;mr12;mr13;mr14;mr15;mr16;mr20;mr21;mr22;mr23;mr24;mr25;mr26;mr30;mr31;mr32;mr33;mr34;mr35;mr35;mr40;mr41;mr41;mr43;mr44;mr45;mr46;mr50;mr51;mr52;mr53;mr54;mr55;mr56;mr60;mr61;mr62;mr63;mr64;mr65;mr66;m0;m1;m2;m3;m4;m5;m0.;m1.;m2.;m3.;m4.;m5.;m6.;m00;m01;m02;m03;m04;m05;m06;m11;m12;m13;m14;m15;m16;m20;m21;m22;m23;m24;m25;m26;m30;m31;m32;m33;m34;m35;m35;m40;m41;m41;m43;m44;m45;m46;m50;m51;m52;m53;m54;m55;m56;m60;m61;m62;m63;m64;m65;m66;mr.0;mr.1;mr.2;mr.3;mr.4;mr.5;mr.6;m.0;m.1;m.2;m.3;m.4;m.5;m.6;m1.;m2.;m3.;m4.;m5.;m6.;lmr;mrx0;mrx1;mrx2;mrx3;mrx4;mrx5;mrx6;mx0;mx1;mx2;mx3;mx4;mx5;mx6;"ntnl bndry";admin_lv=4;"ntnl park";"huge river";"hugest river";"medium river";"small river";"wide river";"hugest canal";"huge canal";"medium canal";"small canal";"big canal";canal;snowpark;Skitour-descent;illuminated;Nordic-Skiing;skitour;platter;ropetow;tbar;goods_lift;runway;taxiway;apron;VFS0;VFS1;VFS2;VFS3;VFS4;VFS5;VFS6;VFS7;usf;cstrn;steps;parking_aisle;res;cob;cobstn;public_footway;public_footpath --route --country-abbr=auo --country-name=australia-oceania --mapname=65340000 --family-id=6534 --product-id=1 --series-name=omtb_australia-oceania_11.06.2021 --family-name=mtb_auo_11.06.2021 --tdbfile --gmapi --overview-mapname=mapsetc --keep-going --area-name="australia-oceania_11.06.2021_omtb" -c D:\openmtbmap\maps\template.australia-oceania C:\OpenMTBMap\contourlines20\australia-oceania\7*.img typauo.TYP