The bug
The 23w43a changelogs say "Mobs cannot suffocate inside" copper grates, but they actually do (they suffocate players too). The only copper grate that doesn't suffocate entities is the default (non-waxed, non-oxidized) copper grate.
Steps to reproduce
Place copper grate 1 block above the ground
Spawn a creeper below the grate
Notice the creeper suffocates
The only one they don't seem to suffocate in is the standard non-waxed copper grate. Everything else, they do