It's just a different opinion of what is standard.
Good question. Have you asked in their forums?
If you follow the link provided by user poster, you will see that MM maps certain other fields to year. And perhaps they don't want to loose any data and create a second YEAR field ...
As MP3tag accurately displays even the second (or more) YEAR field, I do not see any compatibility problems. I do see implementation problems on the side of the other programs.
What I don't understand: you ran into tagging problems because you reached the memory limit and it is still not possible to think of a different workflow?
Did you know that it is possible to d&d files from iTunes directly into MP3tag for editing? With (just as an example) iTunes as the administration program for the whole collection you could pinpoint exactly those files that are necessary to perform your current task. So you might save the MP3tag features and still access your whole collection.
Haven't asked in their forums as I'm new to MM and just noticed the issue when I retested Mp3tag and Audacity is not a primarily a tag editor. It would appear to me that MM and Audacity see the same year field, with Mp3tag being the odd one out, tho perhaps if we considered more programs, the majority vote would come out differently.
I use iTunes for adding lyrics, but I don't like its interface and options for other tag edits. My workflow is the dog here, not the tail.
Again, my recent comments are designed to suggest improvements to Mp3tag, as I hope to come back to it in the future. We can always learn from how others do things.
Hi , I've had this issue several times and always forget the solution ..
thus I'm bumping this old thread to include it .
As the OP states it normally occurs with images ... well most image formats other than .jpg will cause it . I've just wasted a good hour chasing down a .png which refused to be written , obtained a .jpg of the same album artwork and it wrote without question 
Which issue are you referring to?
The original posters issue was the lack of memory due to Mp3Tag beeing a 32 bit software and loading files more than about 50.000.
This is no issue any more since years. For this 32bit-version it was solved first partly by inventing to use a library as an option and later improved by using LARGEADDRESSAWARE in windows. And since version 3.15 there is a 64bit-version which solves this issue finally as long as your computer itself has enough RAM.
Most audio formats allow for both jpg and png images to be attached. But there are some limitations as well, based on total file size maximums. Also, the images need to be flattened, and cannot be progressive (jpg). So for the png image you were having issues with, perhaps there was something that was not suitable for embedding. This is beyond the scope of mp3tag, it is not an image editor.