Thanks for the report! It looks like this is a small place that still uses JJ2's original sprite-drawing code, rather than JJ2+'s. I know there's a 7.0 branch that does move the end-of-level screen into JJ2+ codespace, which will fix the problem; a faster fix could also be arranged if this (32-bit gem sprites in SP/Coop) is something people are interested in being able to do in the meantime.

The only other place in the codebase that has the same problem, from what I can tell, is the black death screen, if you try to have any 32-bit RABBIT::DIE sprites. I'm not sure offhand if there's any current branch that will fix that.