I also experienced the same thing. And I've also written about this issue.
I don't understand the cause, whether it's because of the wrong way to use it or there is a bug in RPR.
I've tried using version 3.0.8 and weekly build version. The result is the same.
Until now no one has answered questions on this issue. My only hope is to be able to use RPR, because Cycles often have problems if I render using an AMD GPU (I'm using RX570).