Hey,
I figure out that my FPS problems just happen when I activated Instant Replay or something else like that (Instant GIF, In-Game Replay) ... When I deactivate this options I can use Radeon Software 20.4.1 and higher without any issues.
When I activate Instant Replay my FPS are dropping every 2-3 seconds while the driver tried to hook to the game (on mouse cursor u see windows loader animation). Thats annoying because I use Instant Replay frequently but cant used with Version higher than 20.4.1. on my RX590.