Short answer: Yes, it's been slipping for some time. Long answer: What is happening is that there are inconsistencies in the UI, not the UX… I know, that's rather pedantic of me to bring up, but there IS a difference. The UI is part of the total experience (UX) one has with a product or service, and an inconsistent UI can lead to a negative user experience. How? By users expecting controls to be a certain way, and then having to relearn a different structure or pattern in a new app.
A more important question that I would make is this: Does it matter? Is Apple being (internally) inconsitent with their app UI a problem? As a "purist" I would say yes, it does, because it increases cognitive effort and can cause potential confusion for users. To play devil's advocate, though, I also have to point out that users don't just use Apple's apps… they use LOTS of third-party apps (the last I read the average number of apps that people have on their iOS devices is between 40 and 60). If you look at all these different apps you will not see many of them follow Apple's UI design guidelines… and this inconsistency has not appeared to cause a lot of frustration with users.
So, to answer the "does consistency matter" question, I'd say it matters far less than learnability and usability… and a significant majority of the apps I have surveyed and used are quickly learned and quite usable.