I’ve tried approaching the branch by the field station in the hedge area to discover the hedge ascent for hours and I still can’t discover it on my map. I’ve tried looking straight up, toggling third person, and approaching it by different angles and it still won’t appear on my map. It doesn’t seem to be a bug on just my specific save, but on new saves as well. 
 

I am on Xbox series X. It doesn’t work in either single player or multiplayer, but recently I’ve just been playing single player. Difficulty is mild. 

I also had the same issue, here's a save with Allie logged out right where the Hedge Ascent is, I've been forward and backward through the area hundreds of times specifically looking for a trigger zone, and it's just not popping.

https://www.dropbox.com/scl/fi/u5p30lt638mgipediezmd/Hedge-Ascent-Bug.zip?rlkey=pvb3knkt2iidifn8jzsjafh7y&dl=1
Let me know if you need additional files.

I'm in the exact same state as OP, and I about died when you fixed the sandbox bug and not this. After the latest patch, it's pretty clear you aren't aware that it's impossible to 100% on a new (or late 2022) save.