Hi @vyacheslavgDISTINCTCOUNT is expecting to return a single value in a calculated measure. What is the end result/visual you need the information in? This can easily be done in a table or other visuals without DAX.
No, you're not missing something - I was. Clearly, I have members with an appointment with BOTH codes hence it's not adding the 13 and 36 exactly. I was expecting 49 but your example shows me what is actually happening here.