Steps:
1. Open an NXP with any levels using graphic sets with Santa Lemmings. At least one of these levels must have pre-placed lemmings.
2. At the main menu, press F5 to Mass-dump the images.
3. Look for the level with the pre-placed lemmings in the Dump folder. The lemmings come out in their regular color instead of Santa.
I have attached an image as an example. This is a very minor error, but just something I want to let you know of.
Assuming mass dumping is still working (I've tested it for levels, but not for images), the newer experimental versions should've already fixed this due to a different implementation for rendering preplaced lemmings on the preview screen. I'll check it at some point (probably tomorrow) to make sure.
EDIT: Tested it already. Experimental version's behaviour is even worse; it doesn't render Xmas lemmings at all, either in mass-dumped images or on the preview screen (however, it renders regular lemmings just fine). o_O Even weirder since, aside from some positioning glitches (which I'm well aware of, I just haven't got around to fixing for the Santa lemmings yet), they display perfectly fine once in-game, and the same code would be used either way (differing only by a line in the theme file that tells it which lemming sprites to load; there is no hardcoding here, sets that use normal lemmings work the same way, they just point to "default" instead of "xmas").
Found the cause of this and it's fixed now. The latest experimental code now renders all pre-placed lemmings correctly, including santa lemmings, zombies, and even zombie santa lemmings.