The only other thing then that occurs to me is that reprojection has a not insignificant overhead, and works best when your GPU is running at around ~70%. The previous advice from WMR on the 45fps reprojection mode was that you’d want to be able to maintain around 60fps without it on for it to work best.
If you are close to the limit, it can fail often and that can cause fluctuations. Artefacts also become more apparent the closer you are to the limit in my experience.
If you can tweak your settings to where without reprojection you can maintain a solid 30fps, then try turning on reprojection and you may find it holds a nice solid 22.5fps reprojected.