That may be with your examples of the Dire Straits album.
But with the tag-informations of the Def Leppard album from your first screenshot I would expect that you only see "Def Leppard" as album name beneath your cover.
It looks as though the player takes the second entry for album:
if you look at the data in MP3tag then, in many cases the \\ are followed by a space character - and in your screenshot it looks like "The Story so far" starts with a space and not with "T".
In "Def Leppard, X, Yeah" it also looks like one space character too many.
Isn't it that in MACUP you can define the displayed data with the ATF strings so that it is not necessary to cramp everything into a single field?
You draw faster than your shadows, the lucky luke of MP3Tag, I’m having trouble following your pace in the responses ... 
Previously, I would resize my files according to the wind, my desire, the time I had, to do it... in short without too much diligence.
Today, I will be able to seriously progress by really mass-restituting to correct only a few albums according to the need.
Yes, there are undoubtedly small errors to correct, but they are only faithful reproductions of mistakes made by my hand. (a space in addition, a tag 'artist' that has not been 'slipped' into the header of the album field on MP3Tag, but the transcription on Wacup is faithful to what I have on MP3Tag.
Yes, with Wacup, there is a way to work with ATF channels, but I don’t know how it works (ah, is it like here?), and that’s one of the concerns about taking over Winamp’s code, there are choices that are not yet ready, including handling ATF channels.
The ATF channels? Is that what I used for these last few hours?
If yes, it’s because I didn’t feel alone to get there that I came here.
Otherwise, well it’s still a completely dark thing for me...
I tested it and MACUP in this view indeed is able to show multivalue fields. It shows them, separated by a comma and a white space behind der comma.
Yes, the "\\" is retranslated with a comma on Wacup, as said much above. On the other hand, I will avoid it from now on, because the " can indeed pose a problem in MP3Tag. finally, this was the case for me. I will therefore prefer something less 'valuable' like the ";" ":" "," "-" and "." which do not seem to have any influence on MP3Tag.
Finally, maybe I am mistaken?
If you want to merge duplicate fields so that they become just one (which is the far more common usage), then apply an action of the type "Merge duplicate fields" and select a separator unequal to \\ like e.g. the semicolon.
Also, it is definitely worthwhile to dig into syntax for the display in your player so that you do not have to clog the ALBUM field with information that really belongs to other fields or other properties.
But that would be a topic for the player's support.
Yes, I take good note of this additional information, even if I have not yet understood everything (in fact I am far from having your expertise in the matter). That doesn’t prevent me from being interested, and I learn.
For Wacup, yes, there will come a time when everything will really become functional.
Report to Winamp, the code is it seems modified to me, and poses some issues on certain features to its developer who, by the way, also does a great job, alone.
Come on, this time, I think I’m going to disconnect from the computer a bit, getting some fresh air will do me good