In the latest Errata release, there are three US Core profiles having MUST SUPPORT reference of Encounter resource. Two of them, US Core DiagnosticReport Note, and US Core DocumentReference, have Reference(US Core Encounter Profile). The last one, US Core MedicationRequest, has Reference(Encounter). 

Suggest to fix the MedicaitonRequest.encounter to be Reference(US Core Encounter Profile)

https://build.fhir.org/ig/HL7/US-Core-R4/StructureDefinition-us-core-diagnosticreport-note.html
https://build.fhir.org/ig/HL7/US-Core-R4/StructureDefinition-us-core-documentreference.html
https://build.fhir.org/ig/HL7/US-Core-R4/StructureDefinition-us-core-medicationrequest.html