I suspect you mean it creates an mp3 with 24000Hz and 32Kbps (not 32 bit), mp3s have a bit rate mesured in kbps, not bit depth.
The unwanted drop in sample rate from the project rate of 44100Hz to 24000Hz when saving as mp3 may be due to the mp3 bit rate selected being too low …
Instructions on how to change (increase) the mp3 bit rate here … MP3 Export garbled - #3 by Trebor