I've been drafting the same type of pitching staff with minimum IP requirement, 1200-1210 for years now and always doing fine with micro managing to keep them almost always at 100% stamina.
For some reason my latest team's staff just can't get back to 100% near the All-Star break, even if I used up all mop up and AAA innings available.
I'm using Royal Stadium which is a ballpark I've used many times before without seeing my staff suffer any important fatigue.
And it's not that I've had any type of extra innings curse, my team is about average in total IP.

Anybody else having this problem? Did I miss something, was there some kind of update on pitcher's fatigue?

The difference is just huge with my last team using almost the same staff in same ballpark!

Yes, I'm drafting 10 or 20 more total innings than I used to because pitchers do seem to burn out a little faster. I don't know if it's a coding change or if I'm just facing better opponents or more hitting teams. I play theme leagues. With salaries rising I thought we'd need fewer IP but so far that hasn't been my experience.

How long has it seemed this way?

I might be noticing that a bit, too, though it could be my using relievers with fewer IP/162 than I'm used to. Starters seem just slightly stressed to. If this has occurred, it seems like it may have about three to five weeks ago, to me.

Hitting park usage has been on the rise throughout 2016. Not an issue if you adjust by drafting better defense, but increases innings required if you don't

That looks like the most plausible... etiological influence.

Then again, maybe it's something else.