Hmmmmm! Only a few other things I can even think of now would be if somebody has tried to reshape the bend in the return pipe that the filter threads on to, and possibly fractured it at the weld where the pipe meets the tank cover. This would tend to spray returning oil toward the vent pipe nipple opening in the tank cover (if the fracture was in the right place). Could even be a defect originating to when the return pipe was welded to the tank cover before the truck was built.
Those pipes are probably of the rolled and welded type and as such do have a seam in them. Maybe the seam opened up a bit when the pipe was bent to shape by the original vendor to Komatsu.
Is the vent still configured to OEM arrangement where the vent cap itself is mounted to the right rear FOPS upright pillar with a short hose leading to the tank nipple?
If someone has moved the vent cap to a location placing it lower than the tank cover (but why) I would expect oil to be expelled like you are seeing even if no other defect was present.
Long shots indeed, but this is an unusual condition so it doesn't hurt to check some unusual things.