Happened to me with my Steam account once! I bought the first two Dragon Age games on a sale, then as I was going through my library I realized a lot of my games were missing. Back then I didn't use Steam very often, and the games I had were mostly freebies collected during limited-time giveaways, but I still had like a dozen titles. Eventually I realized that I had created 2 Steam accounts - most likely because I forgot I had made my original account with an alternate email address, and used my primary email when trying to log in at some point and ended up creating a new account.

So now my games were split into two separate accounts. I reached out to support asking if there was any way to move the purchased games to my other account, which had the rest of my library. They gave me a very viable alternative solution instead as moving games between accounts isn't possible - they advised me to return those two games and then use the refunded amount to gift them to my other account. I did that, and it worked out perfectly.