The following constraint is present in the US Core Condition profile:

where(category in 'http://hl7.org/fhir/us/core/ValueSet/us-core-condition-category').exists()

Nowhere in the current FHIRPath specification does it state that the behavior of the "in" operator can be overloaded to function like the "memberOf" function and if such was the case, then it would need to be documented here:

http://hl7.org/fhir/fhirpath.html#changes

and/or here:

http://hl7.org/fhirpath/N1/#in-membership

I believe that this is an error in the current version of the US Core Implementation Guide STU3 (v3.1.0).

Per this Zulip discussion:
https://chat.fhir.org/#narrow/stream/179166-implementers/topic/Invalid.20FHIRPath.20constraint.20in.20the.20US.20Core.20Condition.20profile

It was noted that the behavior was present in earlier versions of the FHIRPath spec but was removed in favor of `memberOf`.

The US Core Condition profile should be updated to reflect this change.