I assume you know what the employer requires you to sign up for under Medicare because this depends on their coverage and the size of the employer.
Medicare.gov Working Past 65
Sounds like you are doing what you are supposed to do -
Medicare.gov How to Drop Part A & Part B
from the link: Hope you kept a copy of your declining request - dated!
To drop Part B (or Part A if you have to pay a premium for it), you usually need to send your request in writing and include your signature.
1. Contact Social Security
2. If you recently got a welcome packet saying you automatically got Medicare Part A and Part B, follow the instructions in your welcome packet, and send your Medicare card back. If you keep the card, you agree to keep Part B, and you’ll start paying the monthly Part B premium.
You pay premiums for any months you have Part B (or Premium-Part A) coverage. Your coverage will end the first day of the month after Social Security gets your request.
If you’re dropping Part B and keeping Part A, we’ll send you a new Medicare card showing you have only Part A coverage. Write down your Medicare Number in case you need to go to the hospital or get Part A-covered services until your new card arrives.
Good Luck
It's Always Something . . . . Roseanna Roseannadanna