I was wondering what your thoughts are on this. Could the discoloration of the brick above the opening be from too much heat or bad air flow?
Backdrafting or closed damper ?
A well designed fireplace will have the mason looking at tables that list (it’s in the Canadian codes) the maximum of the size of the fireplace opening allowed when standard sizes of flue tiles are to be used. When you have too large an opening versus the flue size, smoke will roll out at the top of the opening as the flue cannot handle all the gasses (smoke)being produced. The solution to this problem is to install an angle iron as a lintel a couple of brick rows under the current opening and fill in with 2 more row of brick. This reduces the height and total opening size usually curing the rollout.
Look for other causes like blockages, incomplete damper opening, etc before undertaking the above.