@andrewilley thank you for the reply and sorry for my delay.
Sure, I can provide an example. I will upload one FLAC file from my library that was like all of other FLACs affected by mysterious glitch. Oh, and let me please call the moment when Poweramp for some reason stopped to see Album Artist tags in FLACs "The Event" to be short (hope it doesn't sound weird).
So, the attached File was ripped by myself from my own licensed CD by Exact Audio Copy program. I used Easy CD-DA Extractor program to set tags in this File. It has different Artist and Album Artist tags: "Artist" tag - Arjen Anthony Lucassen, "Album Artist" - "Star One". Before "The Event" I could see the "Album Artist" tag for it and all other FLACs in all lists and player interface.
But that's where the story started. A couple of weeks ago I added new web-release to my library, and like almost all the rest it had different Artist and Album Artist tags. It was Avatar 2's soundtrack. I have all soundtracks in my library with "Soundtrack (Movies)" as Artist, and their composer's name as Album Artist. So in list of albums, in track list of this album and in player interface It should have been displayed like "Simon Franglen - Avatar: The Way of Water", but for some reason (because mysterious "Event" has already happened sometime in past) it stubbornly showed me "Soundtrack (Movies) - Avatar: The Way of Water". I double-checked the files (all of them were FLACs) on my PC, and all their tags had been in place. I tried to re-upload them to my device with rescan of library, but nothing helped.
I thought, maybe there was some glitch in Poweramp? Let's full-rescan my library! And... well, that was the action that ruined everything. I believe (maybe I'm wrong, I am a poor programmist) that there is a some sort of "image" of music library stored somewhere in Poweramp data. Even since "the Event" happened Poweramp still has displayed all library correctly. But when I committed full-rescan I destroyed the old "image" and a new one was created, but IT WAS INCORRECT because since some moment ("the Event") Poweramp has already stopped to see FLACs correctly. So none of my FLACs has Album Artist tags in Poweramp library anymore. That's when I decided to start this topic.
As I mentioned earlier, the mp3's were not affected. By the way, it created a weird effect: after that infamous full-rescan some of my albums (especially hand-made compilations) that had FLACs and mp3's has divided into two: the first had only mp3's and was shown in album list correctly (Album Artist - Album Name), and the second was exactly the same, but contained only FLACs and was shown like Artist - Album Name!
I am sure that a month or so ago Poweramp has seen FLACs correctly, I added new releases and everything was fine. But then something happened (I got a few updates of Poweramp around that time - that's why I assumed that something in these updates caused all it), and now, with no settings or anything else changed, the same old files after full-rescan are displayed differently.
That's the story... maybe there is indeed some bug?..
01 - Lift-Off.flac