Me and a few other friends noticed that some of our cows stopped producing milk ever since the 'Dynamic Cow' update. I have a group of old cows from around 6 months ago that I use for milking only, they haven't been pregnant or bred from in that time period but still produce milk daily.
The cows I've been breeding and getting pregnant recently seems to randomly stop producing milk some time after giving birth, according to my friend he's sure this wasn't a problem before the update went live.
It doesn't happen for neither sheep or goats, which only strengthens the belief it was caused by the cow update.