http://www.pcgamer.com/a-new-overwatch-update-tweaks-mercy-and-removes-four-maps-from-deathmatch/

Earlier this week, Blizzard rolled out a big Overwatch update that brought major changes to Mercy and D.Va, as well as the new Junkertown escort map that was revealed last month. Now a smaller patch is out that makes some tweaks to Mercy's on-fire meter, and removes four maps from the Deathmatch and Team Deathmatch rotations. 

The Mercy update changed her ultimate ability from Resurrect to Valkyrie, which enhances all of her abilities for 20 seconds and reduces her Resurrect cooldown to ten seconds. That change was apparently causing some issues, as the patch "adjusts" (although precisely how isn't made clear) its contribution to Mercy's on-fire meter.