Column sorting has been broken in some ways for quite a while. Unfortunately I can't put my finger on what's wrong, but if I define a sort for a particular column it often doesn't get applied correctly by Mp3tag when I click on the column header.
This is highly unspecific and I'm not sure whether your post is in any way related to the topic. Are you using the latest Development Build (v2.44d) and noticed that the selection is not moving when sorting (this is the topic) or do you experience any other issues with sorting. If the latter is the case, please try to report it in a separate topic with possibly a more detailed description of the problem.
Clicking the column header will not change the sort order.
Or it will sort incorrectly at other times. I'll try to find a specific case. I find that it's not easily reproduced, as it often depends on the sort used immediately before.
If it happens again, can you try to select files in the file list and resize the window so it gets updated. Maybe the files are sorted correctly but the text displayed is not updated.