using the latest Blender 2.91.2 and RPR 3.0.8 I set the "Transparent Background" option in the Render Settings. RPR renders much faster, but seems to neglect this setting, as the output of a render has still a black background. Output format is with RGBA enabled, enabling disabling world settings did not help.
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).