Except for wav files, all other file formats are not displayed as 24-bit

All file formats other than .wav have to be decoded to be opened in Audition. The default decode depth is 32-bit, as that is the native working format for the audio engine. So yes, your crappy old 128k MP3 files will be converted to 32-bit Floating Point files. For a lot of material this does have some advantages, especially if you wish to manipulate the dynamics and levels, as the FP format has a ludicrously large dynamic range. So what this means is that in fact, there's nothing to fix; it's as designed.

When this happens, your original file isn't touched at all - it's just decoded. If you want to resave in the same format then this is perfectly possible - the 32-bit file can get re-coded to whatever format you want, and you can overwrite your original if you want. Or if you're careless, even if you don't want... (but you'll get a warning).