i first noticed this with alligator gars and sturgeons, so those're the animals i have the most screenshots of, but i've tested it with coelacanths and they suffer from the issue too.
it seems to be a result of their hitbox clipping into the block - their actual model doesn't go inside the block, but their hitbox is so high that it technically ends up in the block, making the animals take damage.
i couldn't find any other mentions of this issue, so as far as i know this is the first time it's been reported...? sorry if i'm wrong, though.
screenshots of the issue occuring, as well as one demonstrating the hitboxes:

I believe this issue was present with Ichthyosaurus in 8.0.0. I don’t recall if it was completely fixed, but if so that fix might apply?

i'm using 8.0.4, so if it was fixed in ichthyosaurus, it unfortunately wasn't fixed in these lads. (if i'm understanding your reply right?)

update: nautiluses also have this issue, due to their abnormally tall hitbox.

i return with an update!
i wasn't aware that living fossils grew, so i didn't think to test with adults, but this issue seems to be specific to babies.
(...or, at least, my adult nautiluses haven't had the glitch since they grew up. adult alligator gars also seem to have a less disproportionate hitbox, so i doubt they experience the glitch, and i'm really Not in the mood for testing every other mob that was affected.)