[mkgmap-dev] Routing across multiple tiles

I'm trying out maps with routing turned on. The routing seems to work fine within a single map tile, it also works fine when I route to a neighbouring tile. However, if I route across multiple tiles then it always fails. Mapsource just spends ages and then draws a straight line whilst my garmin counts to 100% and says their was a calculation error. Am I doing something wrong or is this something we are aware of?