If a belted case does not headspace properly on the belt, there is something seriously wrong with the rifle or the cartridge. That is what the belt is there for (not, as some think, to strengthen the case head), to provide a case suppport (headspace) point for cartridges with insufficient shoulder to use for support, while allowing better feeding than a rim. (In other words, the belt acts like a small rim.)
But a bright line in that position on the case does indicate excessive headspace. There should be no problem in firing a few more rounds and seeing if other ammo has the same problem.