It's not normal. You are not being a ninny. I don't know much more that. You have a problem, but beginning case head separations don't show up on the outside of the case.
Find a gunsmith with the optical tool to examine the inside of your chamber under magnification. My guess is that the inside of your chamber was scored by a chip stuck on the reamer. The brass would be forced into the groove by the pressure of firing and then wiped off when the case is extracted. A bore scope should find that groove if it's there. I can think of nothing else that would give the symptoms you describe on a regular basis. The cases that do not show the mark would be those in which the pressure was slightly lower. I would expect it to show up every time on near max loads in slightly tarnished cases.
I dunno nothing about Sako's warranty policy, but, if I am correct, it's a factory defect and they should replace the barrel.