I started getting these as well. Like the bottom 3rd of the display goes kinda grey/fuzzy after about 15 mins.
The only solution I found was to actually exit VR from MSFS (but leave MSFS still on). Quit link from within the HMD. Go back and enable Link again and toggle the VR mode in MSFS.
So technically, I do not have to restart MSFS. Only restart link but I have to open the debug tool again and set my preferences.
I have reason to believe this is related to the new Link Sharpening since this fuzzy line thing never use to happen. Next step for me is to turn off Link Sharpening to see if it will resolve it.