Hi! Check this answer: Flickering in a mirror UE 5.5 - #13 by looore91
These console commands worked great (5.4, 5.6 tested).
I would also like to add a few notes about the Movie Render Queue. Besides adding the same console variables to MRQ, I set “Render Warm Up Frames” (Anti-aliasing) to “True” аnd set “Render Warm Up Count” “Engine Warm Up Count” to 1024. It took a while to render, but the noise almost completely disappeared in the reflection of the mirror.