Are you trying to edit maps made in the old Zone Builder in UZB? Doing that will break the map and is definitely not recommended without converting the maps first.
There's a good tutorial for how to convert maps made in the old Zone Builder to maps that UZB can edit here: https://mb.srb2.org/addons/instant-binary-to-udmf-tutorial.5215/
If you were editing maps that were made in UZB and you got this problem, try pressing the Tab key, since by default it disables stuff like linedef based slopes and FOFs, and you could have accidentally pressed it at some point.
It should work, but you should make sure you have the "Renumber tags to resolve conflicts with existing tags" toggle set in Preferences > Pasting, since it's set to something else by default and could cause a lot of tag conflicts.
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