Johanna’s level 20 talent Heaven’s Fury can heal a friendly Deathwing. Deathwing should be immune to friendly healing.
They say Johanna can also heal structures. I haven’t seen if she can heal through Ana grenades, but that seems possible.
Ana’s grenade reduces healing. Deathwing cannot be targeted by allied healing, but still has full healing for his own healing sources. As such Ana’s grenade will likely reduce the healing it does.
My guess is the target filter is set incorrectly.