Seems like people are just jumping to conclusions because they are convinced everything needs to be some nefarious corporate plan and constantly need something to be rallying against.
Something is clearly broken (I have 2 codes I can't redeem), the most likely situation is they pushed a code update live that hadn't been fully tested.
PS+ has 10s of millions of subscribers, that will be millions of lapsing subscriptions between now and the new service launch, and there is no way they are trying to deliberately block them from extending the sub (never mind the issues with losing access to games/cloud saves/etc) in case some of them are trying to commit to the playstation ecosystem for years. People here forget that they are an absolutely minuscule sample of the userbase and people who are aware that they can stack subs (and are actually willing to spend the money in advance) are a tiny drop in the ocean; they are not remotely worth the time developing, testing, launching a system stopping them (and thats before you even consider the legal ramifications and negative attention).