I have 2 copies of the game on two different Steam accounts; EA (from early 2018) on one of my old Steam accounts, and current (from November 2019) on the new.
When my second copy asked me for login details, instead of making a new account, I typed in my old details. It’s worked for me before with other games where the account is registered separately, but it was rejected here. No problem, just didn’t want to unnecessarily register a new account.
So, I ended up making a new account, using my new copy, on my new Steam account, using a different e-mail, a different username, a different password. Everything is as expected in-game. Then I login here, and see that my old account’s details, such a posts, badges, are attached to my new account’s username.
And, no, I’m not accidentally logged in to my old account right now, or, if I am, I’m not logged in to it via my old username. This account is now registered to my new username, but that username was never attached to it before, didn’t even exist before a few months ago. The old username has been detached from it.
I don’t necessarily mind that my old and new accounts have merged here, but it’s weird and I’m not sure how it happened (or how I did it, if I did).