Changes between a FOF texture and middle texture offsets in Visual Mode are inconsistent
Ok I was fairly convinced I was able to accurately point out the issues, but as I investigated it further, things got a bit messier than I thought they actually were, so I hope you don't get too lost here.
First, when hovering over a FOF sidedef, and using the arrow keys or holding left click to change it's Y offsets, it will instead change the middle texture from the target sector itself (it does also change that specific side of the FOF Y offset that you looked at in Visual Mode, but it will look unaltered in-game), however, doing the same procedure for the X offset works fine for the FOF, though it also changes the middle texture.
The only way to actually change the FOF Y offset is to open it's Edit Linedef window and change it's offset values directly, which isn't very convenient when we want to use Visual Mode for that. Also, you are unable to reset the FOF offsets through a shortcut too, since it will just reset the same middle texture offsets again.
Second, changing the FOF middle X offset through the Edit Linedef window will change all the sides of the FOF textures in Visual Mode, while the sidedef X offset won't change anything, but both changes do nothing in-game.
Here's the test map, even though it's fairly simple to recreate: udmf_texture_offset_issue.wad
This is the first issue I made in git, so if there are any issues (xd) here, let me know.