I was having the same problem that for some ID3v2.4 tags written by Mp3tag, MP3ext was not able to display the bitrate icon.
As I found out, the reason for this is that MP3ext does not support ID3v2.4 tags or newer, so it is unable to skip those tags. However, MP3ext searches the first 2052 bytes of a file for valid MP3 frames. If the ID3v2.4 tag incl. padding is smaller than that size, MP3ext is able to display the correct bitrate icon anyways, otherwise it is not.
I'm in the process of submitting a bunch of patches to the author of MP3ext that fix this and other issues, e.g. the invalid padding refered to here. Let's hope he accepts them and releases a new version, as MP3ext is supposed to be discontinued.