Tested this with Garodar ( Thilan ) And Firechaos ( Frostshotz )
There's a slight incorrection with the 15% physical reduction, it's normally 13% around there, as an example. I did 77 physical damage, without the buff on. And 74 with the buff on.
Now, with Dark Soul and Dark Apotheosis, both active. I did from 77 damage to 41. It's incorrect, due that mastery affects the reduction and the percentage of the reduction.
The correct way should be 31 damage, and not 41. It's a "major" bug since, the Dark Soul and Dark Apotheosis isn't really affecting each other, or should I say Mastery isn't being affected that much as it should, when Dark Soul is active while Dark Apotheosis is too.
Thanks to Firechaos and Thilan for testing this- Tested on x100 and xfun.