Reports indicate Sony may have disabled PlayStation Plus subscription stacking
However, users are now reporting that they are unable to stack time onto their PlayStation Plus subscription, whether through the PS Store or prepaid cards. Posts on reddit, Resetera and Twitter show that those with active subscriptions are unable to add more time to them, even with prepaid cards. According to PushSquare, customer support has told customers that the function has been "temporarily disabled" and concerned users should keep an eye out for updates on the situation. Over on the PlayStation Twitter, customers have been either contacted through private direct message or directed to a generic FAQ support page.
While Sony has not confirmed why the functionality was disabled or even if it was done intentionally, it seems likely it was done to prevent users from purchasing PS Plus subscriptions at the current lower price knowing it will soon grant them premium benefits.