Currently we have Encounter.reason and reasonCode as must support.  We also have Condition.encounter.  The fate of Condition.encounter is a bit up in the air (as of 2022-02-03), due to FHIR-34553, but the possible resolutions to that tracker are very likely to have encounter diagnoses being represented via Condition.encounter.

I think we should either have MS on Encounter.reason* properties, OR a requirement on Condition.encounter (either MS or equivalent conditional narrative, as proposed in FHIR-34553).  I don't think we should have requirements on BOTH.

Note: While US Core is not based on FHIR R5, in R5, PA and PC made (/are making) some major changes to Encounter.diagnosis and Condition.encounter.  FHIR-16148.   I'm not sure we have enough clarity from ONC about what they mean by "Encounter Diagnosis to really align with the R5 modeling though...