Open trapdoors, which were a barrier to mobs in 1.17.41, are now passable. I started using trapdoors back in 1.16 as walkable fence gates. I could walk across them, but mobs couldn’t pathfind across them. This worked all the way up until 1.18, and now my horses, llamas and sheep are just walking right across them.
Absolutely love the update, though. It’s beautiful.
This is not a bug. It's actually a feature now. Like villagers being able to jump up on their workstation. Sorry to say, but you won't be able to use trapdoors in this way any more.