Moira was perceived (incorrectly) to be stronger than Mercy between late January 2017 and late April 2017.
Mercy/Moira Comp pickrates (PC)
Early May 2017, Jeff made that “Mercy looks fine” post. Which she really did.
But Mercy pickrate kept going up and up and up during May and June and July.
And mid July devs posted the Support balance patch. Because Mercy pickrate had gotten higher than it was back in December 2016.