If Pehkui is installed, new pet mimics don't move and can't be interacted with in any way. It's as if they were told to sit, but I can't open their inventories or attack them, and it seems they can only take damage from suffocating. But if I remove Pehkui and relog into the same world, they start working normally without any issues even if I put Pehkui back. I guess Pehkui somehow disables the behavior of new pet mimics or something? Whatever it is, it means I can't use pet mimics on my current game because I have a few mods dependent on Pehkui.

This is on MC 1.19.2 + Quilt 0.18.6, and I tested it with only Probably Chests and Pehkui + their dependencies. I'm using all the latest versions of the mods for 1.19.2 Quilt.
Probably Chests 0.5.5
GeckoLib 3.1.41
Cloth Config API 8.2.88
Pehkui 3.7.2
Quilted Fabric API 4.0.0 beta.30 + Quilt Standard Libraries 3.0.0 beta.29 for Fabric API 0.76.0

Thank you for the information I will look into this as soon as I get around to updating my mods. I have not done any checks to make sure my mods still function properly on Quilt so that may also be an important factor.

It did occur to me after making this that it could be a Quilt thing. I tested on Fabric just now and while Pehkui still causes new pet mimics to be unresponsive, just exiting and reentering the world is enough to fix it for Fabric, so both Pehkui and Quilt have something to do with this. I'd imagine that complicates things since it's a different mod loader, so no rush. Thanks for responding!