Towards the end of the game when fighting one of the calypsos you fall through the map after the cutscene thankfully if you loot machine that picks up missed drops caught the legendaries. I suggest making sure that machine is empty before fighting bigger bosses. Happened twice once in normal once in tvhm. I suspect it will happen every time on my character
This has happened in other games to me, sometimes the update that is done when you first install a game, is not installed properly.
I deleted the game and reinstalled it.
This resolved my issue.
Remember not to delete your saved games.
Only uninstall the game and updates.
Maybe this might help you as well