If you collect more than 5 Memory Vestiges, they don’t auto-combine into a Legendary Memory. I had 20 sitting in my inventory and nothing happened. I had to delete 16, go back down to 4, and then the next drop finally converted. They don't combine even when you don't have a legendary memory in your inventory. Apparently, this has been in the game for over a year with the release of Final Shape, and you guys still have not fixed it.