Here's another absurdity of the rule. It's based on ticketed cabin.

Last year I couldn't get 3 seats together on a mid-con for myself/wife/kid. I got wife/kid ~8AB and i had some other Y ticket (middle seat). I decided to try to upgrade and trade down - who wouldn't turn down an F seat in trade?

UG cleared and i traded down to 8C. The announcement says "use the lav in TICKETED cabin" - so when i got asked why i was strolling up to the F lav i showed her my F stub. I then asked if she would insist whoever i traded with would need to walk back ~25 rows...