The reason this occurs is that the escort quest (in data/chats/escort-quest.lua) calls game.player:knowTalentType(t.type) to see if the player actor knows the category.
If it is nil, the mastery is changed to 0.8, and then it gives the talent.
Otherwise, it just gives the talent.
knowTalentType() (in engine/interface/ActorTalents) is just a redirect, as seen here:
The problem stems from the fact that the chant talents themselves are in celestial/chant-chants, NOT celestial/chants.
Therefore, since Sun Paladin and Anorithil don't have those categories in their birth descriptions, knowTalentType() returns nil when called on the category of celestial/chant-chants talents. Their mastery is then set to 0.8 when they take the sun paladin escort's talent rewards since nil was returned.
This may require a different solution than was used in the patch (the "passives" function in Chant/Hymn Acolyte).
I do have an add-on
that sort of fixes this bug, but it's not as elegant as the patch tried to be.