black marks on romex and on insulation

not a duplicate of What are these marks on the Romex wire in the wall?, but very similar.

I noticed some black marks on both some romex, as well as the underside of the fiberglass insulation that was on top of it. (this was a piece in a series of quite poorly done splices by the previous owner, feeding a garage workshop.)

After looking at the linked post above, I both cleaned the romex with paint thinner, and also pulled the conductors out. The inside of the yellow sheath has also been discolored and the coating is thinner and more brittle than the other parts of the cable. though the conductors themselves don’t look to have any damage. This is on a 15 amp circuit (at least that is the current rating of the breaker.)

I don’t see any tar in the area, and this was a recently added extension of an existing circuit by the previous owner. (again poorly done with probably 5+ (exposed) splices.

EDIT: Ok TIL that the tar is on the backside of the insulation (that glues it to the paper)… But still odd to see such a long streak, could the cable have just become warm (still safe) and that just softened the tar enough to leak out? The rest of the cables are above the insulation except in this one spot.