In AllergyIntolerance, the reaction only allows to add CodeableConcepts to profile how the allergy or intolerance manifests. Discussions in zulip (https://chat.fhir.org/#narrow/stream/179166-implementers/topic/AllergyIntolerance.20reaction.20-.20reference) show that there is the need and that it sounds reasonable to allow a reference on an observation, as the observation might exist before it is infered that it is an allergy, and that in some cases, observations and the inference of an allergy are made by different people.

Thank you for looking closer at our request.