Hi, I'm experiencing this same problem with the volume of Opus files being extraordinarily low with ReplayGain turned on. I read through this thread and I already manually cleared the Opus Header Gain tags in foobar2000. I scanned ReplayGain tags in foobar (scanned as albums, with a 4x oversample True Peak enabled.) I checked the tags and the header gain tags were indeed removed. When I play opus files in Poweramp with track ReplayGain turned on (I always listen to music in shuffle mode,) the volume is very low. When I listen to the same exact files using other Android music players like GoneMAD or Pulsar, it's the correct volume and much louder. In my estimation, around 10dB louder. I made sure the RG preamp settings are the same for each app (set to 0.) I also followed the directions in the above linked thread, and set the Opus header gain tags in foobar's advanced settings to "Null".
I have attached a sample Opus file for reference.
ID3 tags for the file:
Opus file: https://drive.google.com/file/d/1iOg_yaSVzPuikvEcy0G-QG454KuxAG7t/view?usp=sharing