Some remarks and suggestions

The main reason of trying Mp3tag was that using the 'Case Conversions' option of 'Tag&Rename', gives some trouble. By searching for that option, I found Mp3tag and after installing I've done a short evaluation.
I started with the mp3 rips of the recently bought 5 CD-box '100 Franse Klassiekers'. Unfortunately, getting all the tags filled with the right data was not successful so far.

Because 'freeddb' don't have data for these albums, I was glad that 'Tag Sources' shows a few other sources. And I was very surprised that 'MusicBrainz' not only knows these albums, but also shows an impressing amount of data with many details. Getting the data into the tags of the mp3 files of CD1, was easy and the result was OK (when the files are sorted at track number). But doing the same with the mp3 files of CD2, was not successful because I got the CD1 tag data.

Then I started with adding the folder with the CD2 files to the already loaded CD1 files. The first problem is that the added files are not sorted at track number (while the CD1 files still do).
After selecting all the files, (nearly) all the fields in the 'Tag Panel' get '' and at Title and Artist I changed that to ''. But after getting the 'MusicBrainz' data, I see that the Album and Track are overwritten too.

Apart of these problems, I have a few suggestions too:
#1. A shortcut for 'Save tags and go to next file' or at the bottom of the 'Tag Panel' (at the left), 4 buttons with 'Save and previous', 'Save', 'Save and next' and 'Cancel'. But the current 'Ctrl-S' plus 'Ctrl-N' (after finding that somewhere in the FAQ section) works, but I prefer very much the 4 buttons.
#2. A selectable settings option for 'Don't change the timestamp when saving tags'.
Nearly always I prefer to keep the timestamp of CD ripping when changing tag data.

Problems with the contents of tags sources are not in the influence of MP3tag. You would have to turn to the owners/operators of the respective original web site to see how to get the correct data.

Concerning #1 and #2: see File>Options>General>Tags
The first 2 options (cursor keys save and keep timestamp) should cover your suggestions.

Thanks for your reply ohrenkino.

Tag option 'Preserve …' is just what I wanted.
After checking 'Save tags …' the changes are saved automatically when using 'Ctrl + N', but I still prefer the 4 buttons in the 'Tag Panel'. More intuitive, more choices and very clear, especially for new users.

But the described problems are not related to the 'MusicBrainz' data.
When I'm going to add files and the current files are sorted at (e.g.) 'Track', I expect and want that the added files are sorted in the same way.
When I'm going to import tag data from internet and has set some tags in the 'Tag Panel' to '<keep', I expect and want that these tags are not overwritten by other data.

The 4 buttons are more or less there on your keyboard in the shape of the cursor keys.

The intended workflow is fairly easy: web sources are there for you to get from "invalid/no tags" to "tagged".
Perhaps you can adapt your workflow that you first use the web sources and then modify the data for your needs.

I repeat that the 4 buttons are much easier and more clearly than using keys. Especially when you did not use the program for a long time and/or normally use a lot of programs (what makes remembering the right keys difficult).

Regarding the problems or bugs, I've found an easier way to fill tags of CD-box files and that is using TagScanner. With that program it is not necessary to process all CD's of a box set at the same time. It is namely possible to select only one of the CD's (or even one or more tracks). Furthermore, all the available tags can be (de)selected and the deselected ones are really skipped. In my opinion Mp3tag should do the same, if a tag is set to in the 'Tag Panel'.

Note. During ripping of a CD, the files get (for sure) the right track number and therefore that tag should not change when loading tag data. If e.g. Title and Artist got the wrong info, it will be very difficult to recognize that if 'Track' got a wrong number too.