From the PTR notes:
Divine Star (Talent) and Halo (Talent) now deal reduced healing beyond 6 targets.
Developers’ note: These spells previously used a complicated, invisible AoE-capping scheme for healing. This is being replaced with a clearer one where the amount of healing decays on each target after the 6th target is healed.
Divine Star’s heal cap is now independent on the outgoing and return trips.
Halo (Talent) healing reduced by 20%.
Developers’ note: Combined with the above change, this is still a net healing increase on large numbers of targets.
TLDR Is Divine Star & Halo used some convoluted system before that did a radius check around you and if there was 6 or more targets it made the healing do tooltip_amount * 6 / number_of_targets healing per target. This is why you’d notice Divine Star/Halo doing half as much healing in a big raid than the tooltip said.
Unknown about the new formula, but if it works like the AOE cap than it’ll be a big buff.