The license (this might not be the correct legal term) for Shuffle iT to make and operate a Dominion program goes until the end of 2021 (IIRC it was initially until end of 2019 and then got extended for 2 more years).
That means a 12 sub month starting today (January 1st, 2021) would reach into a time when it isn't sure if Shuffle iT can provide the program. So it only allows subscriptions that end before that time. In 3 month the 9 month subscription option will vanisch, a.s.o.
If the license is extended again, the longer subs will apear again.