The Mercenary's Disciple does not give 750 Combined Rating on Mythic while it should.
Example steps to reproduce this:
1. Have a character with the Mercenary and any other companion (e.g. the Deepcrow Hatchling) on Mythic
2. Equip Baby Deepcrow's Presence and observe your Ratings
3. Swap Baby Deepcrow's Presence with Mercenary's Disciple and observe your Ratings
Example images:
Equipped Baby Deepcrow's Presence:
Swapped Baby Deepcrow's Presence for Mercenary's Disciple:
As you can see, all the Ratings are dropping by exactly 750 each.
Tested on Live and Mimic with the same results.