I get the same thing on my RX 6900 XT.
Try adjusting your custom resolutions with different base fresh rates like 60Hz 75Hz 120Hz 144Hz 200Hz or 240Hz.
Also under windows advanced display tab make sure you set the FPS rate to the max supported per the resolution you want to run.
I've had this windows 10 display feature act like a conflicting display driver before. Its a pain in the backside and likes to reset itself to 29Hz after every windows update.