Mules and possibly horses and donkeys also will walk under trees that will suffocate them. Rather then damage them they should just be blocked from entering area. It is causing animals to die around the edge of plains and even tethered near trees. I was unable to tell if it is when they rear up or just move, but then rearing is putting them in a worse situation.
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I think this is separate from whether leaves cause suffocation. I am guessing that it is intended that leaves suffocate. But mules should not kill themselves this way.
I don't think this bug shouldn't be marked as a duplicate of MC 46765. If leaves causing suffocation is intended, mules shouldn't enter blocks that cause them to suffocate.