21.3M Medicare beneficiaries are enrolled in Medicare Advantage Prescription Drug plans which includes medical and prescription drug coverage. That means that when a prospective enrollee is evaluating plans, they are comparing the plan "bundle" as a whole, and not the health plan and the prescription drug plan separately.

Reference: https://www.kff.org/medicare/fact-sheet/an-overview-of-the-medicare-part-d-prescription-drug-benefit/

The formulary IG describes the plan shopping scenario as part of its use case "Shopping for Health Plans" https://build.fhir.org/ig/HL7/davinci-pdex-formulary/use_cases_and_overview.html#use-cases

However, the IG currently does not have a way to logically represent that a Formulary 'Payer Insurance Plan' is connected to a PDEX Plan Net 'Insurance Plan', which, would be the case in the MAPD scenario. This means that a prospective member using an app/workflow leveraging a combination of Formulary and Provider Directory APIs would not be able to find plans that have their drug on formulary.