Thank you for your comment Alex B. It's encouraging to get some feedback. Unfortunately, I still have not be able to identify the cause of this issue. It remains a mystery for me. Hopefully further testing will reveal.
I hear what you're saying about the problem not being a common one... However, I've discovered the issue on two completely separate machines, one running XP and the other running Vista. One is accessing the MP3s directly from the local hard drive, and the other is accessing the MP3s from a 3rd server machine. Perhaps, I could format one of the machines and only install MJ12, see if the problem persists. But first I'll attempt a less drastic solution...
Regarding the comment: More than one software application handling the MP3 data/tags
All MP3 tagging on my music collection is done manually in MJ12... and only MJ12. No other MP3 tagging tools are ever allowed to access the MP3 collection! However, I do utilize 3rd party software tools, like Tag Runner. But... I actually duplicate the MP3 data and keep the original data (MP3s) safe from harm. Meaning that I'll copy a portion of my MP3 collection, run scans on the copied files, use that information to manually edit the original collection in MJ12, and delete the copied files immediately after. I find that the 'automatic' tagging tools are useful, but only for some information like YEAR and LYRICS. Therefore, MJ12 is the only software that ever gets to edit/tag the original music collection.
Regarding the comment: ID3 version
You mentioned the ID3 version of the tags, I will look into the this. However, why would YEAR field be the only field with an issue, wouldn't all the tag fields have an issue is the version was not compatible. And why would the tagging of the YEAR eventually work, after renaming the year once or twice. Sometimes I have to rename the field a 3rd time, but cannot remember having to do it more than 3 times for a particular song.
Encoding
One particular MP3 that reverted back to the original year and had to be edited twice before the year remain was had the following profile: LAME 3.92, 320kbps, 44100Hz, MPEG1 (Layer III), ID3v2.3.0.
I've also run a scan on this specific file (copied version) in Helium Music Manager and it appears to be in excellent health.
YEAR appears to be an ID2v1 tag field.
Date vs. Year?
Which tag field should one use for the year release of a song, YEAR or DATE?
It appears that one edits the other.
Answer to question: I assume that you have not changed any of the default library fields to not write a file tag?
All the settings are on default. The only customization is the reorder of the column headers, which should not cause this issue.
Run auto import in background: On
Update for external changes: On
Regarding action window > Tag
I prefer to tag the MP3 in the list directly (rename / F2).
But yes I have seen this, and have tried to tag the YEAR field here as well. The same thing happens... sometimes the year sticks and sometimes it doesn't.
More information about the issue
When changing the YEAR field of an MP3 directly in the list (or in the tag panel), sometimes the YEAR field will revert back to its original value. This occurs for both original YEAR values that are filled or blank. (Described in more detail above).
If I edited a whole bunch of song years and then watch the list, on refresh, I can actually see some of the years revert back, whilst other remain corrected. Terrible annoying!
Still looking for a solution
I'll continue editing the music collection. I'll report when I have issues. Perhaps we can get to the bottom of this.
Perhaps someone could run some tests? I wish that I had the one of developers of MJ12 sitting right here to see the issue with their own eyes!