For me it is more than the rear speakers. When I select a video clip on the timeline and then open the 5.1 Surround/Stereo Panner, adding a keyframe either manually at the bottom of the Panner or changing the mode in the dropdown at the top of the Panner (which adds a keyframe) reduces the volume of the whole clip in all speakers substantially.

The last communication I had from Corel they said: "Our engineering team has been working on your issue - you provided excellent detail that helped us investigate. There have been some improvements in internal test builds, but the improvements are not released yet. As soon as they are, I will advise."

That communication was December 16, 2021 just after the 25.0.2.276 version release where it was still not working. I just updated to the new release (25.1.0.345) and it is still not working. Must be a huge problem.