Please use this thread for feedback and bug reports regarding this encounter, or related issues. Noting the nature of your group (Mythic guild premade, PUG, etc.) is helpful, as are any logs. Thank you to everyone who has come out to help test!
This issue should be resolved in future 7.3.2 builds. When engaged with a raid or dungeon encounter, Lady and the Child will now consider all monsters in that encounter as valid targets. This change was also made for Purge the Wicked and Path of Flame.