As per OP is a Series X so it is on the installed SSD. The main SSD not external.
Zero issues with other games.
It was a install on a new console end of December. I’ve started a new SP server, losing significant progress and time spent in game in the process and the bug appeared immediately.
Reinstalling the game won’t do anything. Reinstalling, updating drivers etc never fix problems when the issue lies with the game.
Since some corpses are insta disappearing when I turn away it is most likely something to do with the clean up routine which was changed to fix performance problems months back.