These changes seem to be aimed at the recent healing tuning that is coming with Patch 12.0.5. The three meta healers are getting nerfed, which would make this encounter more difficult for players that are already working on it. Because of this, it seems like Blizzard has nerfed some of the healer output checks on Mythic L'ura to compensate and not have players feel like the the fight is suddenly more difficult. That being said, it's odd that Blizzard has done nerfs to Mythic L'ura, especially so soon after they recently nerfed the encounter and not any other boss including Mythic Belo'ren, which also has a high healing check that many players are using the meta compositions for.
Additional changes to 12.0.5 next week:
DUNGEONS AND RAIDS
March on Quel’Danas
Midnight Falls
Dusk Crystal health reduced by 10% on Mythic difficulty.
Glimmering damage reduced by 10% on Mythic difficulty.
Shattered Sky damage reduced by 10% on Mythic difficulty.
