Glancing at the type hierarchy in the current build https://build.fhir.org/fhir.ttl I notice a few odd things. Why do Dosage.doseAndRate and other elements of Dosage have type Element, instead of BackboneElement? Actually, shouldn't the only direct subtypes of Element be DataType and BackboneElement? https://build.fhir.org/types.html These are mapped directly from StructureDefinitions + ElementDefinitions, so this is just another way of looking at those.

I see R4 has other subtypes of element besides BackboneElement and DataType: https://hl7.org/fhir/R4/datatypes.html Were some accidentally not updated when this changed in R5?

probably, yes

Can these all be changed to types of BackboneElement? (Except xhtml as Datatype, maybe). I think that wouldn't be a breaking change. I can make a JIRA ticket for it.

they should be, yes