The length and bitrate of an MP3 file are not user-editable fields. Rather, they are calculated by Mp3Tag or taken from the file header. These fields are information fields in Mp3Tag. The fields in Mp3Tag are _LENGTH and _BITRATE and are not offered in the extended editing dialog.
So if you use the editable field LENGTH (without underscore) it is not the normally used information field _LENGTH.
If you look at the field with the heading Length in the configuration of the column view, you will notice that the field _LENGTH is behind it.
There is also the possibility in Mp3Tag to create an editable field LENGTH. However, this is not equivalent to the _LENGTH field used in the column view and is not displayed there either.
However, your problem with individual files indicates that the corresponding files are corrupt or not real MP3 files. So you should check them with the usual test programs.