I don't know the PD connections with mannitol.

But I do know that all of the sugar alcohols are also used as sweeteners, and as such they can be laxative depending on how much is consumed. Sorbitol is used in hospitals for constipation, but many people get a laxative action from it from foods where it is added to improve consistency and offer a sweet taste.
Hershey's had it in their chocolate kisses with cherry filling.
Most diabetic candies contain sorbitol. (I've had reactions to the Hershey's and I called them up to complain and they said they don't have to label their product as "containing sorbitol" until a serving reaches 50grams).

People consuming sugar free mints and gum can get diarrhea, gas and bloating too.

Mannitol is used IV also in people with brain swelling from trauma or surgery. This is medically supervised.

Perhaps your dose could be lowered? Or you divide it up over a day in multiple dosing. There are 3 teaspoonfuls (15ml) in a tablespoonful when measured carefully. If you use 5ml at 3 separate times, it might lower your side effects some. I think this is worth a try. The sugar alcohols are considered osmotic laxatives, as they draw water from the blood into the lumen of the GI tract. I have to wonder then how much is actually absorbed into the blood at any one time.

Xylitol, mannitol, and sorbitol can be found in many foods and other orally consumed products. There are sites that suggest they are responsible for the epidemic of irritable bowel syndrome with diarrhea.