Discribing:
When mob is walk around, they will be avoiding to drop in the holes . When there are a circle of trap doors around the first level of the hole, mobs will drop in.
The way to make this bug appear:
1.Create a new, creative-gamemode world.
2.Dig a hole in the size of at least 2*2*2.
3.Put wood trap doors around the first level of the hole, if you cannot understand, look at the photo.
4.Use spawn egg to spawn some mobs around the hole(They mustn't be attracked by you), and wait.
Result:
The mobs falls in.
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Comments

Mobs seeing trapdoors as full blocks is an intended mechanic.
Thank you for your report!
We're tracking this issue in MC-50556, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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Duplicate ofΒ MC-50556Β which was later resolved as "Works As Intended".