I have been noticing this issue since around Feb 2017. All my MP3s before that where I used $rg2sc() to set COMMENT ITUNNORM to ReplayAlbumGain for every track, imported into iTunes just fine. Song Info->Files->Volume shows the same value for every track in the album. However, anything I imported after Feb 2017 (using the same function/method) has "random" volume values for the track, which I assume are the ones that iTunes SoundCheck assigns if nothing in COMMENT ITUNNORM. I verified this by blanking out COMMENT ITUNNORM and ITUNNORM and re-importing the album after deleting it, and I still get those same random values instead of the consistent ones. (Note that supposedly COMMENT ITUNNORM is used by iTunes for MP3 files and ITUNNORM for AAC files.) For me, this issue is independent of whether importing an album for the first time or later.
This has been driving me crazy, I can never get it to use the values I set any more. Then I saw this recent post which seems to explain it:
https://community.mp3tag.de/t/convert-replaygain-values/8069/45
So it appears to me that in 2017 there was either a change to MP3tag to encode the volume slightly differently, or there was a change in iTunes to interpret it differently (or a combination of the two). I can not find any way around this. It sounds like $rg2sc() needs to be changed.
[Edited to point to actual reply within thread that shows issue.]