Thanks for the bug report. I think I know what's going on and it should be fixed in the next update.
I've experimented with some premade levels when I was trying to figure out how to do the tutorial, and found that limiting the number of creature parts was more frustrating than a fun challenge. Something open-ended could be fun though — I've been working on the level editor for the race mode in the next update (spoilers) so should be able to start thinking about challenges after that. :)