It should be possible for you to test the files with the linked tools
prior to tagging them with MP3tag.
This should show whether the files are ok after the treatment with Audacity.
And then we can see if there are any settings that you can modify in MP3tag to achieve the desired results.
If that is all too much, then please supply a fresh file untreated by MP3tag so that perhaps others can have a look and see what may be wrong.
Honestly, if your problem were a widespread one, the forum would be buzzing with such reports. And as from the distance no-one can look over your shoulder, we have to rely on your support.