The buff applies to the FvS before the healing happens, but after it does for allies.

We noticed this today by comparing Healing Amp on two characters, and found out that I was still healing more with Beacons than our tank was despite him having slightly higher HAmp than me.
Thought this was a bug with the Beacons at first, and after further testing, found out that Shining Light (the HAmp buff that applies when you use Beacons) applies to the FvS before healing, while it applies after healing to everyone else.