Please share some process settings with us and answer these quick quick questions. Does the mold have any valve gates? Does the mold have multiple gates to fill the part? Can you confirm that the water circuits and flow directions, plus temperature settings, are the same? You are NOT using intrusion or flow molding to partially fill the part? No changes to the mold?
I feel that your melt front advancement has changed and I believe that you have multiple gates. One of these gates is not flowing (blocked by tramp metal, wood or even cardboard) and creating this flow line.
Rick