I have just started a snapshot world in 20w45a with world type Amplified, and I'm hosting to LAN (bad wifi, so there are some connectivity issues lol). I'm barely into the world and I found some squids which are completely invincible to damage from the player, and seem to have no AI at all. (I can still nudge them by running into their hitboxes, but they just drift until their momentum runs out, at which point they revert to an upright position.) I found some more squids, and some fish, which act as normal.
(I do have a video, but due to the aforementioned wifi issues, I may not be able to provide it for about 2 weeks.)
This is probably the same issue as MC-203893 (and MC-203893 is probably closely related to MC-203880).