They tend to do that in Harvest when they get to the centre.
At first I thought it was having issues with all the dead creature models in the same area but, nah even after cleaning up the corpses after ourselves there's some weird concealment issues that doesnt seem to affect the scorpions, that happens to those mummies as soon as they get into the hallway that lets them attack you from what mechanically appears to be stealth (given the bonuses) without revealing themselves.
Best not to let those mummies pass through any doorways.
Didn't realise it was happening in hoteps temple to Sobek as well.