You are right with the last statement.
I have an opposite experience. I was never OPUPed on TPAC but 2/2 on my last NRT-ICN-NRT segments.
Last Sunday the GA in NRT was really confused with seat assignments both in Y and F on NRT-ICN flight. She messed up OPUPs and two PAX had the same seat. Despite of 4 empty seats in F, local women with duplicated seat assignment was sent back to Y, while US-originating PAX got the OPUP.

And interestingly, Y was not full .