If a bat flies under a block, sometimes it takes suffocation damage.

You'd expect the bat to not take suffocation damage as it flies under blocks.

This doesn't happen very frequently, but I noticed a bat taking damage at what appeared to be the same rate as suffocation damage as it flew under some stone in a cave. There was no lava, and the water in the cave was well outside of the bat's hitbox.

To replicate this, I suppose you could summon a bunch of bats in an otherwise safe cave and wait for it to happen? I only saw it once and I've not heard the bat take damage since I've begun writing this report so I'd assume it's very uncommon. It's also worth noting that I'm on a realm, so it's possible that server lag played a role in this.

I've attached a screenshot of the cave I was in. The cobblestone is where the bat was suffocating. It was originally stone, but I replaced the blocks with cobblestone to highlight the spot.

This isn't a very important issue but it's something that's likely unintended, so I figured I'd report it.