Let a floor be marked for removal.
Let a pawn be removing that floor or on the way to do it.
Have a second pawn right click on the floor, and no context box occurs, which means you cannot prioritize.
Thanks for the bug report. You can still prioritize, you just can't override a person you've already prioritized to do the task. However I do think this is a bug and have confirmed it is active in 1.3.3164. I'll get a report into the devs!