I am using Windows 7 and was using Mp3tag 2.83d. It's been some months since I had occasion to look at replay gain values in Mp3tag so I can't say exactly when the problem started. However, I have lost the ability to display both track and album replay gain values for M4a files.
I use Winamp to calculate and save track replay gain values. These show up in Winamp's File Info dialog but not in Mp3tag 2.83d. Now both the tag panel and the list column show nothing. By contrast, my Mp3 files always display replay gain values as expected. I did not test any other file types.
When I ran the view filter expression %replaygain_track_gain% PRESENT AND %_extension% IS M4a it showed no such files in any of my music folders, whereas in fact virtually all my M4as do have replay gain values.
I tried reinstalling 2.83d from the latest download. At first, I thought this worked as I could suddenly see the missing replay gain values. However, after a reboot all M4a replay values disappeared again! At this point, I was rattled and confused.
At my wits end, I decided to reinstall an earlier version of Mp3tag (2.77). I chose this one only because I had a copy on hand. It worked! All my M4a replay gain values are now shown. The view filter expression that I quoted above now returns 1604 files. Whew!
This bug disrupts my workflow because I select files to be processed for replay gain by using a view filter that only shows files that lack values for %replaygain_track_gain%. I then create a playlist from those files that I open in Winamp, which does the rest of the work.
This is one of the rare bugs that I have encountered with Mp3tag. In other respects, I would call this program a triumph of German engineering! I really don't know what I would do without it.
New Jersey, USA