I think you’re right! Gonna try and reproduce it in a minute:
A stray farm designation has become attached to a unit, no idea how and I’m unsure what it wants. I will not give in,
@sdee bear with me and my non technical surmising but I’ve figured out how to reproduce it, to me it seems as though removed designated areas aren’t being flushed from the game?
This was my latest attempt and is probably the easiest to describe.
Place multiple stockpiles, as you can see I placed 6 (you don’t need to place 6, I think I got it to work with 3 or 4, interestingly enough with just 2 it wouldn’t attach to anything, but would just stay overlapping the existing stockpile, I might need to retry this):
Then proceed to delete all but 1 stockpiles, I deleted 2 - 6 left the original one. Then go to place another stockpile, as you can see the game names this stockpile 7, even though there’s technically only 2:
If you then remove stockpile 7, and keep pressing the zone tools button, stockpiles you have already placed appear in random places, or simply overlap the already existing stockpile: The bottom stockpile in this picture isn’t a stockpile, I can’t click it. The same with the stockpile to the left in the second picture.
Tell your settlers to harvest some wood, then keep pressing the zone tool button and at some point whilst you should have your zones attach themselves to an item, I originally thought it was attaching to workers but I believe it’s attaching to logs, although you can see it is attached to the guy chopping the tree down, maybe it’s attached to his axe?
It seems as if the stockpiles aren’t completely removed, linger about the place and then attach themselves to an entity, that’s my layman interpretation anyway 
This image shows better how removed deisgnations, both farm and stockpile, seem to randomly overlap, the only stockpile I have control of in this picture is the one with the transparent wood, everything else had been removed.
Also more pictures of logs with stockpiles
DO NOT REPRODUCE, YOU WILL ANGER OUR WOODEN OVERLORDS: