This is on the Experimental Build:
Using a controller, enter the Perks screen for the hero listed at the top of the Party screen. Place the cursor (right-pointing hand) over the first Mastery skill (such as Calibretto’s Attack Power Mastery). The details box correctly explains that Mastery skill.
Now hit the RB to highlight the second hero. The cursor remains at the same position (pointing at the first Mastery skill position), but the details box displays an explanation of the second hero’s first Perk (such as Gully’s Reinforced Knuckles) instead of its first Mastery skill.
This bug occurs regardless of which hero is first selected in the Party screen.
EDIT: The same behavior is seen when swapping between a hero’s two Perk trees via the Y button.