I have 3 ideas.
Its not common to see wear on the main bearings. Especially #5. Keeping that in mind I'd first suspect an assembly issue. For instance the bearing may not be seated well in that location because a spec of dirt found its way under the bearing during assembly. Or perhaps a drop of some chemical found its way there during assembly.
BUT the wear is clocked on axis with cylinders #5 through #8. That's probably not a coincidence. This could indicate several problems with one of those cylinders. An ignition issue, a lean cylinder, a bent connecting rod, a balance issue, a crankshaft damper issue. Inspecting the other 4 main bearings may help in determining which cylinder in that bank has an issue. BUT ... the exposed copper area does not appear (in the photo) to have signs of mechanical wear that would go along with these things. By mechanical wear I mean visible ridges, lines, grooves, craters, etc.
When bearing damage occurs without signs of mechanical contact with the crankshaft journal I think its normal to assume its caused by cavitation or corrosion.
Did you guys notice its a fully grooved main bearing?