Hi Martin and Omar,
yes it’s so strange: i clicked into the file again, and selecting the faces on the uv map, it really does seem “shuffled”.
I also simply clicked and dragged the uv faces a bit on the crazy ones, and it seems it’s not really connected:
on the left i was pulling the face on the normal one, it stays connected, and on the right im pulling on the faulty ones, and it comes apart:

It is connected on the actual mesh though, (i tried)
ps. the way i understood your analogy with the 2 planes is: the 2 planes aren’t a result of mirroring/flipping, since 0 and 1 are not connected…
hence they just arent the same topology,(which is why copying uv doesnt work) while mirror modifier + uv tends to be safe (i use it a lot actually).