Is it absolutely forbidden to sleep on one’s stomach according to the Sunnah, or just disliked?
For our understanding of what the ‘Sunnah’ is, please see related answers on the site. It should help you understand the Sunnah of the Prophet (pbuh).
I hope that you will read some of our previous answers as they will help you to better understand the answer that follows.
Sleeping in a particular position is not a Sunnah. It would seem that the Prophet (pbuh) disliked the sleeping on the stomach as for the exact reason for this saying of the Prophet (pbuh), this can only be determined once we take up a thorough study of all the various texts of the narrative under consideration.
I hope this helps.