So? Now I have to stay with the old version to maintain a good quality for my MP3-files?
A lower bitrate doesn’t necessarily mean worse sound quality. It does mean a smaller file so maybe it’s doing a better job of compressing.
However, re-compressing MP3 does tend to degrade sound quality. If all you are doing is normalizing there are applications that can make volume changes without decompressing, such as MP3directCut (or MP3Gain for volume matching (There is a limitation that you can only make volume changes in 1.5dB steps without decompressing.)
But is the new version of LAME in 2.4.0 so much different from the version used in Audacity 2.3.3?
As I said before: I used in both versions the same settings and I was really happy with the results of Audacity 2.3.3.