G'day Nathan,
That is true actually, I, too, have found that potato starch, in particular, is a problem. Because the potato starch doesn't come off with the same consistency as other soils, I've been pre-washing these pots as a result. With pasta I haven't found it to be an issue. Maybe because I put a teaspoon of olive oil in the water? There is usually no starchy residue to speak of. For cooking pots of any kind I usually pick the longest, hottest cycle and make sure the soil faces the water spray head on and solids get scraped off - no pre-rinsing, of course.
Rusty racks in a three year old Miele and they say that it is the tablet formulations? I wonder if that's been picked up by consumer review websites? Are other dishwashers similarly affected? That shouldn't be. Miele is a premium brand and this I would lay at their door step. So they recommend that you use powder, but gave you a sample of 'their' tablets. Let me ask, "what are they worth on the open markte?" Sounds to me like a bit of back-handed marketing, if you ask me.