All mobs I've tested float above the iron grate as seen below:

The mobs tend to spin around when on it, and seem to try and jump up onto the grate when pathfinding past them, but are blocked as if the block were higher. The hitbox for the iron grate seems to be different for different mobs, as far as I've tested, pigs are lowest, then sheep cows and chickens, then all other mobs are 1 block above.

This issue doesn't seem to affect the player.

Using Minecraft 1.16.5, quark r2.4-311

Latest log: https://pastebin.com/hBDLyAA6

Reproducible in 1.16.5 r2.4-311