You know what would help troubleshoot it, get a different drive and see if you still have the same issue. NVME drives are extremely cheap these days. Just get a small one that's big enough to hold windows and some applications. Heck, until just recently I ran my entire main system's Windows installation on just a 256GB drive and I had a TON of programs on there. Windows itself only takes about 15-20GB for a clean install and I use a lot of major software like the full Adobe suite, the full Microsoft office suite, tons of audio and video recording, rendering and conversion software, game launchers, software for ripping, authoring, playing and converting movies and music, professional automotive shop labor and repair management programs, software for working and communicating with my QNAP NAS, a bunch of web design software and plugins, plenty of various hardware monitoring and testing utilties and on and on, and really never came very close to using up the full 256GB on that drive.
Even now, I'm only using a 512GB OS drive, so for just testing purposes it's an inexpensive option and of course it's always usable after testing if you end up not leaving it in.
There are a butt load of 256GB drives you can get for between 20 and 30 dollars, just to see if you have the same problem with a different model than the 990 Pro. It might not give you the result you're looking for but at least you could eliminate the idea that it might be due to the drive model and move forward. I'm thinking it may potentially have something to do with them being NVME drives which have an entirely different mechanism for control than SATA drives do. What exactly about that which could cause this, I'm not sure, but the more you can narrow things down the easier it will be to figure out.