The only thing I can think is that you might have been in a special assistance seat and were moved because someone with a health problem needed that seat, also, on those international flights it might have been a bassinet location?

I haven't done too many long hauls in the back lately, so I'm not sure if CO is selling bassinet locations or special assistance seats as "premium seats" -- maybe someone else on here can opine on that?

In the last 4 years that I have been CO Gold/Platinum, I've probably flown 150+ segments...not once have I had my seat location moved on me; and I can't remember the last time I flew in coach where I wasn't seated in an exit row.

I did have one problem with a seat assignment the other day, but it was on UA metal. I made a seat assignment in the ELR section of the plane, but because my reservation hadn't ticketed on the UA side it cancelled my seat assignment.

Sounds like lightning struck twice ,or maybe four times for you..I wouldn't expect it to become a pattern. BTW, Welcome to FT