This is happening again in 1.16 Pre-release 5.
Confirmed in 18w49a. See 18w49a file.
I'm getting the same thing. Snapshot 13w25c. Unresolved. Very noticeable with baby chickens as they get stuck within the hitbox of the corner fence and cannot even be touched. Sometimes when they mature they end up on the other side of the fence. Really annoying when you are breeding lots of chickens because almost 100% of the time the baby chickens will get pushed into a corner. Seeds won't lure them out. Hitting adult chickens won't cause them to flee from the corner. It's as if they think they are in a hole with no escape.
Not sure if suggestions are being taken on this issue. I've been tracking it since 2018. It looks like there are attempts to split this into multiple issues, but the solution would probably be a more holistic one that resolves all of them. Rather than fixing individual areas where the Woodland Mansion cuts the terrain, it looks like there needs to be a terrain buffer around the Woodland Mansion structure. This would be a zone around the structure with generation rules specific to the structure area, so that it can attempt to blend with the existing terrain. The Mansion itself is a pretty rigid structure. It doesn't blend well if it is just dropped into the middle of the forest. With a buffer area, there is an opportunity to create a more natural-looking entrance and keep the structure at a decent y-axis level, since it would have finer control over the terrain around the mansion than the default terrain generation.
Currently, the Woodland Mansion is one of the more neglected structures in Minecraft and doesn't really "fit" into the game's aesthetics given its harsh placement rules.
I know I'm placing this comment on a bug report. This may be better as a feature suggestion (like a Woodland Mansion rework). Let me know if there is a better place for this kind of comment.