Same issue here, scoured the internet and tried out pretty much all the fixes out there including:

- underclocking/volting
- driver versions
- ram changes / psu changes

Eventually binned the 6900 XT and went back to trusty NVIDIA 1070 I still had laying around. Conclusion from the past years seems like it's an AMD driver related issue conflicting with something of certain FFXIV patches.
If you enjoy playing final fantasy XIV in particular, just avoid AMD cards. For everything else it works great sadly.