Fireplace Profile Seam
The profile seam is a continuous bonded collar joint. The front brick wall is bonded and attached to the main chimney structure with a collar joint of bonded grout. R606.3.1 mentions the thickness requirements for collar joints. A little trivia, in masonry, the bed joint is the horizontal joint when bricks are laid on top of each other.
The head joint is the vertical joint where bricks are bonded next to each other. The collar joint is the mortar that bonds two sections of brick to each other. Normally, a brick façade wall is 4 inches nominal thickness and if you wanted an 8 inch nominal thickness wall then you would form two 4 inch walls bonded to each other with a continuous collar joint. The profile seam is a continuous bonded collar joint that typically separates and deteriorates because of differential settlement between the chimney and the house framing and thermal break down of the collar joint. The best way to repair this issue is with C199 refractory mortar.