Preheat an outdoor grill for high heat and lightly oil the grate.
Season chicken thighs with salt and pepper.
Cook the chicken thighs until no longer pink in the center and somewhat charred on the outside, about 7 minutes per side. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).
Let chicken cool until able to handle easily, cut into strips, and put into a large glass bowl.
Stir pineapple juice, bourbon whiskey, soy sauce, garlic cloves, brown sugar, and black pepper together in a saucepan over medium-low heat; stir until the sugar dissolves, bring to a simmer, and cook until thickened, 15 to 20 minutes.
While the sauce simmers, melt the butter in a small skillet over medium-high heat. Saute mushrooms in hot butter until softened, 3 to 5 minutes. Stir into the simmering sauce.
Pour the mushrooms and sauce over the chicken strips and toss to coat.