I'm a fairly new fATPL holder but was previously Crew.
In my honest opinion, I have seen the Pax Info List have some odd choices of seat closure. Often you will see a seat blocked for take off that was open on a previous sector with the assumption usually being it is a M&B issue though that may be an incorrect assumption on occasion.
However CC are not generally trained in the why's and where for's of load control and their choice of seat allocation, we were simply asked to enforce it. That is made all the more difficult when pax do not comply.
At LGW I found myself on a 737 one day asking a gentleman to move seats to the one he was allocated (about a 10 row change from over the exit to the near aft row), he refused stating he was a TRI for another 737 operator and that I was being difficult. Thankfully my flight crew came out and moved said gentleman.
Was the M&B out with him out of place? honestly not having gotten further than CRM in my company type & line training yet I really am not sure, I've seen people moved from first class to last row before but this is a far more obvious choice than on a stubby 73. However At the time I was doing my job as trained, as was the young lady on your flight.
There is of course every chance she was misinformed or feeding you a line for whatever other reason, however, like the above TRI your place is not to judge but to comply with the polite request and, if you feel hard done by, speak to the Senior after take off about moving back.
Not having a go but we get enough of a hard time from those entirely ignorant of aircraft, airlines and aviation, when a fellow cloud dweller is aboard it is nice to feel you are on the same team at least.... now I best not rant about the positioning Delta girl who insisted her seat was upright and why had I woken her for landing - she was in a flat bed....
