I don't know why in cases like this people would often very quickly go to the conclusion it's mainlanders, but frankly out of my 10+ years of extensive flying globally and within the region, with quite some domestic Chinese flights, I have never seen mainland Chinese pushing their ways into J/F cabins on the plane. well, Chinese might not necessarily be used to the queueing culture yet, but at least they seem to know their seat numbers and the ticket they bought and don't push for something they didn't pay for and feel perfectly ok being in Y.

instead, I have seen multiple cases of Caucasians/non-Asians doing it, and yes, poor FAs having to deal with these situations. in one situation I remember seeing the FAs doing a head count and finding one more person than what should have been, and finally discovering one of the pax didn't belong to the cabin. he argued that he misread his seat assignment or sth like that, and after some fussing around moved back to Y.

and of course I have seen way too many DYKWIA cases of people seeing a loooong F/J boarding queue (since all MPOs including GR are eligible), and going straight to the front of the line and showing their J boarding pass (but to be fair to them, if they didn't fly CX/KA enough they probably didn't know this weird rule and never expected a F/J line longer than Y). anyway, in many cases I have seen, the gate FAs dare not to say/do anything (esp when it's Caucasians) but to accept that one or two boarding passes and let them go...but in some cases when it became serious enough (i.e. it becoming like 2 queues), I have voiced out and said both to the FAs and the pax there's a lot of people queueing already and the queue is way back there. I clearly remember in one instance where the pax then showed me his J boarding pass and said, in a DYKWIA tone, 2 words, "business class" - to which I immediately pulled out my boarding pass and showed him and said "first class".