So I totally believe you because I got the chance to see you on with one of the mobs, and when I looked at the mob's fields it was missing one that should have been there.
But every time I test it it works fine.
SO! Help me find out what circumstance is causing it. I've spent a few hours on it with no luck so far. But if you can come up with any specific situations that differ for when they stay vs. leave, please let me know! There is *definitely* a bug.