I can confirm that there is a solution how to fix this bug - just download and run OBS studio while playing WOW. Flickering disappears. Or play in windowed mode, it also fixes it.
I can add that the reason why flickering and fps drop disappears while using obs is that obs activates small orange dot (microphone using indicator) in wow UI. That indicator fixes this bug. So basically you can join any discord voice chat or use any application that uses microphone and flickering is gone. Maybe that will help developers to find this UI error that causes it.