My guess, cavernous sinus infection.
Forehead sensation is supplied by V1 – Opthalmic. Trigeminal ganglion makes up the lateral border of the cavernous sinus so I’m assuming that pressure within that space would cause paresthesia of the 3 branches of the trigeminal nerve.
From your treatment picture it looks as though you dissected as close to the area as possible (pterygomaxillary fissure) and placed a shunt to allow drainage and oxygenation of the tissues.
I’m not entirely sure on the pathway of infection into the cavernous sinus but I would assume the bacteria would go from the pterygomaxillary fissure into the inferior / superior orbital fissure to gain access??
Anyways, not sure how accurate that is, but that is my guess.