DetectedIssue is already set up to point to all of the implicated therapies.  Pointing to MedicationRequest to detectedIssue as well creates a bidirectional link, which isn't supposed to happen in FHIR.  There's no reason to have MedicationRequest.detectedIssue.  If you have a system that 'owns' a DetectedIssue and the issue doesn't live independently of the MedicationRequest, you could still have a contained DetectedIssue that points to the containing MedicationRequest.