I have a bunch of sound effect files which are all 320kB/s CBR 48kHz and any that are smaller than 1 second in length display a bitrate less than 320k. The smaller the file the lower the bitrate reported. e.g. a 236ms file shows 84kB/s.
File Explorer, foobar2000, MediaInfo, Exiftool, VLC etc all report 320kB/s correctly on the small files.
MP3 Diags shows the correct values in the header.