Diarrhea does cause weight loss.
However the weight loss that diarrhea causes is basically just water weight and not actual fat weight loss.
The diarrhea causes you to lose water in your body and when that water is lost you lose some water weight so you may weigh a bit less than you would have before the diarrhea.
But the weight loss won't be significant and the weight loss from diarrhea will only be temporary and not permanent.
Diarrhea actually dehydrates you so you don't want to have diarrhea just to lose weight as that is very unhealthy.
When you have diarrhea you need to stay hydrated and replace lost fluids and if the diarrhea gets worse you need to see a doctor or go to a walk in clinic or the emergency room as you may need an IV to get fluids back in you when you're too dehydrated.
Stay hydrated and eat foods that are bland while you have diarrhea and even take some anti diarrhea medicine to help stop the diarrhea.