I ran into the following problem:
All my tracks already contain all the necessary ReplayGain tags. However, after synchronizing with my Music.app library, Audirvana reads these tags only partially, and does not use them when playing tracks. After clicking on Recompute ReplayGain, Audirvana overwrites the old tags with new values that are already displayed and used normally.
OS X 10.15.1, Audirvana Mac Version 3.5.24 (3554), Music.app library syncronization mode
Here is an illustration of the problem: