I have the similar issues here. After updating to Big Sur 11.5 on my iMac 27", 2017. The external Samsung Display (2 x C49HG9x) flickers. :/
ausdemnichts
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution