Hi Gerd, great news!
It will crash if any hgt file is missing I have them all, but there is no HGT for sea area, missing files is a standard case.
I did not check if the flags in the tdb file are set correctly now I didn't check either, but tdb created by mkgmap v3999 works correctly for maps with DEM.
My currennt understanding is that mkgmap should do the calculation of the DEM resolution based on the levels used to create the rest of the map. I think it should be like that and some Garmin maps follow this rule. I have tried to create DEM with pixel size ratio like 1:4:16:64 for levels 24,22,20,18. It didn't work correctly in Mapsource. What works for me is ratio 1:2:4:8. I think some of unknown fields in DEM header could contain this ratio. It is something to investigate further.
-- Best regards, Andrzej