Why, over the last month, are Specials now showing up on the Android app calendar?
Looking at the website the same Specials do not appear on that calendar.
Specials are hidden in settings.
I do not really use up next, but I do tend to try to watch all the specials when possible for each series. This does not seem like a bad thing. 
Not Up Next, Calendar.
It is when you don’t watch them and they are disabled in settings.
I generally tend to agree with you. I’d always ignored them because so many were either short recaps, interviews that didn’t appeal to me or 5 minute previews to an upcoming season.
Until I discovered some items I watch have “Christmas Specials” which in reality represent the first episode of the next season. Some BBC/ITV series are notorious for doing this.
But if that’s not the case for your series then I’d think it was a bug if they’re appearing for you in your Calendar.
I understand what you say about Christmas specials but certainly not specials I want to watch in my series
Blake’s 7 from the 70’s
The Apprentice from a good few years ago
Both suddenly appearing.
Most definitely a bug because not all specials are appearing.
That would frankly irk me no end seeing those specials.
FWIW this issue doesn’t manifest itself in my iOS app. And I have several shows that do have some specials coming up. And some where I missed specials in my watching habits.