I just encountered a less severe variant of the bestiary loading loop issue. It happens when defeating a Mimic (no. 8, Cradalios Canyon rabbit hole path) and then viewing its bestiary entry. At least I don't require an immediate fix this time. There's so much to do, you'll have this issue fixed a hundred times until I decide to bother with Mimics again. :D
Okay, I have figured out what is causing it this time! I'm going to take a wild guess and assume that If I don't have a battleback in the area you encounter something that the book will break! I am going to spend all day and fix every single area that I overlooked (I made the rabbit tunnel quickly and forgot to put the simple battleback there for the dirt caves haha)
It's a great idea that you intend to check every area. When you fight Rabbid Bandits (no. 256) or Bright Riders (no. 288) in the Rabbit Tunnels, the bestiary loading issue occurs, too, even if you've encountered these monsters elsewhere before, because the bestiary entries are "overwritten" with a different battleback, which makes me think your guess is correct. Man, I miss the days when checking the bestiaries in RPG Maker MV JRPGs simply made the game crash. ;)