I was worried about trapped glass (or other debris) because of the numerous black spots visible on the threads and the sudden start of the damaged area, which is what you get when something gets crushed and rolled along a thread groove, it disrupts any surface treatment and exposes virgin metal which then cold-welds, tears a bit out which adds to the jam and so on. The fact that thread and cap are both made of the same material may not help it resist this galling.
Are those black spots corrosion, if so that could be a failure starter, especially if there are high local pressures due to profile mismatch, poor geometry, fit or surface finish. Noting that the you say the parts were "made to fit" without gauges, this seem quite likely!