Originally Posted by
Goldorak
Remarkable ? absolutely

Common ? certainly not, it's extremely rare. In any case, just a subset of the fleet has P and with only 4 seats each.
To be expected at all in certain circumstances ? not to be expected, but it can happen if J is overbooked and there is still a seat in P.
Last time it happened to me, it was in 2019 at the gate while boarding AF A380 IAD-CDG (the great red light & beeeep at the gate with message "inconsistent class" on the small screen

. I was already Ulti at that time.
Right, P seats are few and far between! I wonder if in the case reported by our lucky fellow, the fact that they had just status-matched from BA helped. Like, welcome to Flying Blue! Look what incredible wonderlands we’ll take you to! You can only decide to remain with us, no?! …but shhh, don’t get used to it 😉