Atonement bug in zones w/Legion scaling tech

Summary: Atonement is not calculating the proper amount of healing in any zone where scaling tech is effective. The effect is more extreme at level 100 than at level 110.

Expected behaviour: Atonement should deliver healing equal to [0.4*(1+Mastery)] x damage dealt by Penance, Smite, or Shadow Word: Pain. So for a character with 20% Mastery, Atonement should heal for 48% of the damage dealt by one of those spells.

Actual behaviour: The above holds true in areas such as Stormwind and Stormshield where no scaling tech occurs.

However, in Legion zones and in dungeons, Atonement is dealing significantly less healing than it should.

Example 1: Questing in Stormheim
Level 100, 20.62% Mastery, expected Atonement healing of 48.25% of damage dealt
Warcraft Logs: https://www.warcraftlogs.com/reports/HJB8wQ72djrkY4yq/#type=summary&boss=-3


Spell........Damage dealt........Atonement heal........Observed %
Smite...........17034................3624................21.28%
Penance.........16281................3464................21.28%
SW: Pain.........4425.................941................21.27%
Atonement is healing for 44% of its expected value


Example 2: In the Neltharion's Lair dungeon
Level 100, 23.84% Mastery, expected Atonement healing of 49.54% of damage dealt
Warcraft Logs: https://www.warcraftlogs.com/reports/vpJGPyBdbzfDRaQX/#type=summary&fight=1


Spell........Damage dealt........Atonement heal........Observed %
Smite...........17034................3720................21.84%
Penance.........15752................3440................21.84%
SW: Pain.........4259.................929................21.81%
Atonement is healing for 44% of its expected value


Example 3: In the Darkheart Thicket dungeon
Level 110, 28.05% Mastery, expected Atonement healing of 51.22% of damage dealt
Warcraft Logs: https://www.warcraftlogs.com/reports/DM4jqQhTz3FJp6dg/#fight=2


Spell........Damage dealt........Atonement heal........Observed %
Smite...........55550...............22646................40.8%
Penance.........44439...............18117................40.8%
SW: Pain........13888................5662................40.8%
Atonement is healing for 80% of its expected value


Further info:

Mastery's effect on Atonement appears to be working properly, e.g. if I increase my Mastery by 12.5%, I also increase my Atonement healing by 12.5%.

It appears as though the game is using an incorrect value for the damage dealt by our spells. The damage displayed to me is accurate for my spell power, but whatever damage value the game is using behind the scenes to calculate Atonement's healing appears to be wrong.

I checked a previous log of the Heroic Ursoc raid testing and Atonement was functioning properly then, so this bug is relatively new.