Unearthed Arcana: The Mystic Class contains the most recent version of the Mystic class (v3).
One of the psionic disciplines listed is Bestial Form; one of its psionic effects is Bestial Transformation, and one of the transformation options is called Tough Hide:
Bestial Transformation. As a bonus action, you alter your physical form to gain different characteristics. When you use this ability, you can choose one or more of the following effects. Each effect has its own psi point cost. Add them together to determine the total cost. This transformation lasts for 1 hour, until you die, or until you end it as a bonus action.
Tough Hide (2 psi). Your skin becomes as tough as leather; you gain a +2 bonus to AC.
Unearthed Arcana: Eberron also contains the Warforged race, which has the Composite Plating trait:
Composite Plating. Your construction incorporates wood and metal, granting you a +1 bonus to Armor Class.
For a warforged mystic, while the Tough Hide transformation is active, would its effects stack with the benefit of the racial Composite Plating trait? In other words, would I have a +3 to AC from those two things while transformed?
I can see arguments going both ways for this, so it is unclear to me.