corrupted data in filestream table
There are basically 3 options:


An app or query is changing the data
Storage is becoming corrupt (which can be due to the OS, HW, storage, etc.)
You've hit a bug in SQL Server


It sounds like you've ruled out #1 (although I'm going to come back to that in a minute - but let's rule out the other stuff first.)

To start ruling out #2, run DBCC CHECKDB on all your databases. I know, you're worried about the filestream data, but it's possible that you're starting to experience corruption in other parts of the database, too. Check all of the databases for corruption, and if anything pops up, stop troubleshooting filestream and start troubleshooting the bigger issue instead. Here's my corruption troubleshooting checklist.

To start ruling out #3, open a Microsoft support case. It's $500, and they work the problem with you until it's done. (Normally I'd say to patch SQL Server to the latest cumulative update (CU) first, but I don't think I've seen any filestream corruption issues in there.) The MS support case may not be a fast fix, but I'd wanna get that process started sooner rather than later. Because filestream is a bit of an edge case feature, it's entirely possible that you've hit a bug that other folks haven't hit yet.

Those two are pretty quick - so if they don't turn up anything, then it's time to refocus on #1 (which would still be my lead suspect.)