Seems pretty standard with all mergers. Seniority is defined as whatever your union negotiates during the merger.

I jumpseated with a 19 yr AA FO who had been stapled twice. The CA was in both of the same mergers, but since he was 1 yr senior to the FO at the time of the first merger he was able to upgrade to CA. So while the FO had been stuck in the right seat for 19 years, the CA had been left seat for 7 years. Because of a 1 year difference in hire date before the first of 2 mergers. You get what you negotiate, but sometimes you're lied to by your own team. That 19 yr FO had documentation going back almost 2 decades of every time he'd been screwed over and flat-out lied to prior to votes. He was pretty bitter, to say the least. The CA had nothing to say other than "he's not wrong".

That's 99% of the reason I never applied at AA. There was a lot of angst going on there from the mergers and I didn't want to be part of that. SWA has angst for sure, but it's a different kind of special that I can deal with.