Just had some serious problems with this, sadly forgot to save diagnostics.
I've used Tdarr for years, and never had an issue, and this wasn't related to Tdarr for me either.
For me it was the docker container for a project called Kaizoku that was doing it, I moved my files off the user share and directly to cache and pointed Kaizoku to that instead, and the issue was completely gone.
I would point it back to get it definitely confirmed, but after days of testing the operation that made /mnt/user die I am quite confident.