Nope – it’s a bug… an infrequent one, but a bug nevertheless. It’s been reported several times but I suppose QA could not replicate it (or there are multiple causes). Whatever the case, not much more to be said except… make another game.
(At least, it wasn’t THE greater rift where you cleared tier 70 solo for the first time… and see the primal at your feet but can’t pick it up).
The bug is listed as #6 here: Player List of Known Issues
Best of luck in your games !