Sorry I missed this when you posted it! I stumbled across it while looking for previous dirty flag problems for About changing an image mask, so I've added it to the issue I filed for that one. There's a low-level LCDBitmap_clear() function that does set the flag (and which the C API clearBitmap() function calls) but for some reason the Lua version uses a different function which doesn't. I need to go look at the commit logs and try to sort out what happened there.
Thanks for the report and, again, apologies it flew under the radar for so long.