Upon game launch, the game will sometimes within a few seconds freeze up then the screen will flash on and off rapidly (attempts to screenshot this failed). 

When the game has not frozen within the first few seconds, upon world generation the screen will occasionally flicker, alt-tabbing has the same affect of causing rapid flickering. 

This is only occurring in 1.21, If I load forge 1.12 up to go to my modded minecraft worlds this issue does not occur. If I switch back to 1.21, the game still freezes then flashes or just occasionally flicker then full on flash.