After more extensive testing, I can now reliably reproduce this bug. It happens when exporting WAV and MP3 audio. I have not tested the other audio formats.
I have a simple project with one track and one clip. The FIRST time I export audio after opening the project, the Clip Gain is included. The SECOND time I export, the Clip Gain is ignored. I am doing nothing other than opening the project, then exporting the same audio twice.
If I move the clip forward or backward in time, the next export is fine, but the one after that, again, ignores the Clip Gain.
Bouncing to Clips and/or Flattening Comp sometimes resolves this, but not always. Even after bouncing or flattening the Clip Gain is sometimes ignored, but not always.