I'm playing on Prominence 1, a 1.18 modpack with this mod in it, I have this issue of when I used a key to turn a chest into a friendly mimic, I just had a creature that didn't do anything but sit there unlovable and unaffected by gravity. Was able to kill it via suffocation though. Modpack team told me to come to you, so here I am.

Also had this problem. Seems like the chest is in sleep-mode. It worked for me by making the chest a wild mimic first and then giving it a friendly-key to make it a pet