Clarifying information
If the Medicare beneficiary is not enrolled in a Part D plan (PDP or MA-PD) on the date of service allow the prescription drug cost toward spenddown. It doesn't matter why the beneficiary is not enrolled. The reverse would also be true if a Medicare beneficiary is enrolled in a PDP or MA-PD on the date of service, we would not allow the prescription drug cost, whether self-paid or not, because the drug is covered drug under the beneficiaries' Medicare Part D plan.
Enrollment in Medicare Part D is voluntary. A Medicaid individual could tell Medicare that they are "affirmatively declining Medicare Part D coverage" but this would not make them eligible for Medicaid to cover their prescription expenses. We could, however, apply the costs they incur for prescription drugs towards a spenddown liability upon confirmation from SSA that the individual had affirmatively declined coverage.
Since Medicare is primary payer for drugs covered under Part D and not Medicaid, there is no prescription drug coverage available to full benefit dual eligible individuals, including those not enrolled in a Part D plan—for (1) Covered Part D drugs; or (2) any cost-sharing obligations under Part D relating to covered Part D drugs. This means that if a CN or MN Medicaid individual with Medicare chooses to "affirmatively decline" Medicare Part D coverage, Medicaid will still not pay for any prescription drug costs that would have been covered under Medicare Part D or any Part D cost-sharing.
Disenrollment from a current Part D plan will not prevent Medicare reenrollment in the same or a different plan. To prevent reenrollment individuals must get their Medicare records documented with their statement that they are "declining prescription coverage under the national Medicare Part D prescription drug program".