There is no faa rule about which lav to use. But there is a state dept one that passengers are not allowed to congregate in groups around the toilets nor anywhere in the aircraft." This all happened post 9/11. I think some airlines and FA have used the congregate rule to the lav rule. The following is a suit a first class passenger did against Alaska when the first lav was full of economy passengers.
https://onemileatatime.com/lawsuit-a...avatory-usage/

But if the airline has made such a rule and the FAA has approved it, then it sticks