This is NOT evidence of DEW or anything similar. The false beams in that ceiling show a seam right down the middle. The false beam burned away leaving what looks like a laser trail. The reason the adjoining wood did not burn could be that it is sealed with poly and the beams were not. Also, the beams appear to be rough-cut lumber which would burn easier than the other wood.

Also, if you look at the top-down view of the exterior, there is no burn line.