Yes, some meds might not be fully absorbed.  Diarrhea is associated with fluid moving into the gut, instead of from the gut into the body.  Most types of D also have faster movement of stuff from end to end, so time for absorption is reduced.  But it depends on the drug(s) in question ... there are different mechanisms for absorbing them, some of which are not affected by D.

Even amino acids can be blocked from absorption by chronic liquidity. People with lactose intolerance or fructose intolerance don't absorb tryptophan, which leads to low serotonin, and many doctors respond by prescribing SSRIs because they think low serotonin is the primary cause of the anxiety/depression/IBS/diarrhea... really, the IBS/diarrhea causes the low serotonin, which then causes the anxiety/depression.  Give them a hammer, and soon every problem is a nail.