On your Sgt. Pepper example, I'm not making those kinds of changes. And if I did, they would be very minimal. Further, the tracks in one library are 99.9% the same in the other as it was a clone.
Here's an example of what I'm seeing. First, the original MP3 song which was in the original library. As you recall, the songs were scattered among a variety of folders and not in the proper structure (Artist/Album) - which they now are. As part of the work, I used various scripts and ended up with this proper structure. As well, I used the Export/Import script so that the lossless version replaced the MP3 but the metadata should have been there. Sadly, I never checked, or perhaps those I did check had it on as this is not on all tracks (missing fields). Here's the original in Windows Explorer in the folder it was in.

And now here's the Lossless version of the song in Windows Explorer now after using E/I.

I don't understand how the data was lost? Now I did create a new iTunes library (Volume1), but I kept the mapped drive (Y:) with the Artist/Album music as well and have checked there and the data is missing as well on songs such as the example above.
So I'm perplexed as to how the data was lost. It appears that maybe a bug in E/I script at some point? (You'll recall you made several updates along the way so maybe something happened there?). Or maybe it is in the registry as you pointed out to try but my gut still says that's not the issue.
So I'm at a point where I've spent several days putting this data back on songs manually and am only up to maybe the mid-B's (Artist) so it will take weeks to update these songs. It's very frustrating since the data was there to begin with.
I noticed also that there are a lot of MP3 files that I didn't have the CD files to convert to lossless are missing the Album Artist or other fields - but I don't think many of those had Album Artist on them so that makes sense. That was a later application of metadata when discovering that Plex uses Album Artist first for some reason. But it still doesn't explain the loss of other data such as the capture above.
My original library is the mapped Y: drive as I said. I cloned that library but used a script to change the location to Volume1. The original library points to the mapped drive and the volume1 (main one I use now) is pointed to the NAS.
I did try using E/I this morning on a song to see if moving the same song (without all the data) in and out might work, but as predicted, it didn't work.
Something fairly significant happened in some step along the way to lose data on some, not all, songs. And the data that's lost might only be the Album Artist for example, whereas other songs, like the capture above, lost everything but Artist/Title.
Is there a script you have that can somehow take the data in iTunes when you view "Get Info" and confirm that the data is there, and somehow "write" that to Windows Exp.? Or perhaps some other trick you know that might work? Something that forces it in there or am I simply going to have to bite the bullet and go through all these one by one?
THANKS!!!