I’ve seen the same thing many times but never bothered trying to figure out what is causing it. Would be interesting to know. You mention using AIG models. So have I been but just replaced AIG with FSLTL and will be interesting to see if that will make any difference in this regard. Which would then make you think the issue is with some of the AIG models.
I’m sure someone else in here knows more about this and what is actually causing it.