Running the Opus files through Volume Analyzer results in the same ReplayGain Offsets, so for this file it is -18.7 again.
If I use the 'Preference > Devices > Settings>level the volume' setting, the resulting levels end up far too low as well.
Under the Tag Inspector is
The only place I see a ReplayGain value for Opus files in MusicBee is in the 'Track Gain' colum of the column display.
The displayed ReplayGain value is being applied to the TagInspector of Opus->Mp3 converted files and functions as expected, just with the level being far too low.