Experimenting with trading again, here's my guess (until proven otherwise)
I reckon there's 3 rules:
Light blue - SM Origin <-trade-> SM receiver = OT reflects origin, based on origin language
Light green - USUM Origin <-trade-> USUM receiver = OT reflects origin, based on origin language
Pink - SM <-trade-> USUM [or vice versa] = OT Sun variant, based on origin language
The way rule 3 (or what I'll like to call crossing the Ultra/Non-ultra OTxReceiver barrier) acts is though both sets of games are different generations...
Also, it appears the 'Korean' interaction is real, however I personally reckon that the report from the Redditor came from them observing rule 2 jumping to rule 3
(if you take a look at my table, the last 3 trades were the same Mew, and the OT changed when it crossed the Ultra -> Non-ultra OTxReceiver barrier. )
That means there's likely yet another invalid character or combination in Korean OTs (lol)
I think there's enough trades here for you to test and prove/disprove.
(And before anyone jumps in, yes, some origins are not possible for the events. I'm testing what happens to a mon with said origin with invalid OTs.)