This issue originiated from a Zulip chat where GrahameGrieve requested an issue be opened (link)

Observation has a few search parameters which use the 'as' type specifier, such as: (Observation.component.value as CodeableConcept). The 'as' documentation says the left hand operator, in this case Observation.component.value, must only have one entry, otherwise an error should be returned.

Each component is only going to have a single value[x], but Observation.component is used if multiple results are included, so the left hand operator is basically always going to have more than one entry. We're seeing lots of errors being thrown on blood pressure Observations, which have two components, each with a single valueQuantity.

This is an issue with the Observation search parameter FHIRPath expressions, they should be updated to use a different identifier, possibly ofType(). An alternative solution would be to redefine 'as' to allow more than one result in the left hand operator.