This old bug has to do with the walk flags for a combo:

A common bug can arise from the fact that enemies will ignore the top two 8x8 walk flags. If a combo has the bottom two walk flags walkable but the top two not, an enemy on there could still go up depending on the combo above. This can cause problems, but should be fixed soon (if it isn't already in one of the the current ZC betas).
It still happens in Build 846. I read it in the ZC Wiki article about walk flags, by the way.

Plus, this bug has been around for a long time... I also noticed a little typo in the article, so I took the 'is' between 'This' and 'can' out of the quote.