reddit user KeyMaster89 reports:

After using it I'm unable to remove anything from my action bars. I don't know if it's important but I have it set up to only remove things by holding "Shift" then dragging in/out of the bars. This only fixes after a "/reload".

Thanks for the report! I think I've run into this too. It doesn't happen all the time, but it's real. I'm looking for the cause.

WORK AROUND: after this happens, I can fix it without a reload. I drag a UFO onto the stuck button. They swap places and then the problem goes away.

Just to add, I have the same issue occasionally and also use Shift to unlock items on the bars

Hello, came here to report this issue too.
To help on reproducing, I got this bug with bartender 4 + ufo
I use one of my bars (bar 3) as a possession bar, (in bartender > state configuration), and i tried to put an UFO on it while dragonriding, and same thing happened : impossible to remove the UFO, but also it seemed like anytime i was trying to use Keybinding of bar 1 at the same button position it was trying to use this flyout on bar 3 instead and overriding my keybind.
Removing all ZUFOs macros after disabling UFO then reloading UFO didn't fix the issue, and moving spells neither, only the workaround mentionned by ScottIngram did, or I would have had to reinstall the addon completely.