It's a mistake that it doesn't appear in the patchnote, but yes, it has been confirmed as a feature of the latest update.

It concerns only legendary items, not epic.

Why did they do that? They explained that it was because legendary items - especially the mono-legendaries - were OP: they made the girls' level and grade irrelevant, as well as raising your battle stat.
With this nerf on legendaries, a balance should be attained between equipments, battle team (girls level and affection grade) and boosting your stats.

This is a fairly understable move, once you realize that a mono-stat legendary item was around 50% stronger than the strongest girl of your specialty, at max level and 5 stars.

They did that to make the game less pay to win: if they kept the game as before, the game would highly favored the paying gamers who spent lots of kobans in epic pachinko and got several legendaries.
Now, the advantage of such tactic is more limited.

It was the right thing to do, imho. Their only mistake is to not have wrote it in patchnote and explained why.