Hi Christian,
For me, the stream that is showing the problem is actual when using the A2DP (but probably not noisy as yours, just with undesired processing), but I haven’t tried other configurations yet. Even though we’re both, probably seeing some automatic gain control, I think we’re seeing it through different scenarios.
My problem description can be found here, if you want to have a look: NREC Enabling/Disabling flag
But shortly put into words, on my case, is the DSP that seems to be applying some noise suppression even when I disable it. Which seems to be very similar to the problem you mention where your headset volume is going down and then back up. Have you tried feeding a more flat signal (e.g. white/pink noise)? Because in your case it seems that the algorithm is trying to compensate for the noise, so it ends up affecting the main signal as well. (regarding the pops, I have no explanation. Are you feeding into via analog or digital path?)
Sincerely,
João