In December 2017, the Centers for Medicare and Medicaid Services (CMS) issued revised guidance on the issue of having your monthly premiums automatically deducted from your Social Security benefit check, noting,
"[Social Security] could take up to 3 months from the time you request premium withhold before you start seeing premiums withheld from your Social Security payment." [emphasis added]
CMS also added some clarification about paying past or unpaid months, noting that,
"[d]epending on when the plan gets your enrollment request, premiums might be withheld back to the start of your enrollment in the plan [as noted above]. In cases where premiums weren’t withheld from your Social Security payment until 1 or 2 months after you enrolled in a Medicare drug plan, you’ll get a bill for the months your drug plans premiums weren’t withheld. You’ll need to pay your drug plan’s monthly premium directly to your plan. Your Medicare drug plan will let you know If there’s difficulty withholding premiums from your Social Security payment." [link added]
So it is possible that the Social Security administration will not withhold back premiums and instead, your Medicare Part D plan will send you a bill for those months that were not paid through your Social Security benefit check. In such a situation, you need to pay these unpaid premium directly to your Medicare plan.
See also: Why am I not allowed to have my Medicare Part D premiums automatically deducted from my monthly Social Security check?
(sources: U.S. Department of Health & Human Services and "Withholding Medicare Prescription Drug Premium from Your Social Security Payment" (Revised December 2017 & May 2019), CMS Product No. 11400)