Kevin,
I am a technician for SurfPrep abrasives and I work on sanding machines for a living.
That mark is a very classic belt splice mark. This is usually most apparent with cloth belts using a thick tape splice without properly skiving the backing to produce a thinner joint.
Once the grit on the joint is worn away it is impossible to get rid of the mark entirely.
This is why we use a zero tolerance lap splice on our paper wide belts. Once we go into production on cloth belts we are going to properly skive our joints so we have zero tolerance on those too.
There are a couple things you can ask your supplier for. Grit side tape on the finer belts will help. The wavy joints are simply a way to widen the area of the joint to try to hide the marks. They can show belt splices marks the same as any other tape splice.
You will see less of the marks with the platen running. Try to sand something with the platen pulled up out of the way, then stop the head and run the piece under the head and lower the platen until it barely touches the part. This will get your platen working with your drum and hopefully hide some of the marks.