The diffusers do get discolored within a few uses, because the polish is burning off, even without water precipitating onto them.
Some precipitation is common in the beginning and with more use, this will stop. It also depends on the stove. Glass stove tops cause a little more precipitation that gas and coil. But again, that will eventually stop.
Placing the pot on the empty stove for 1-2 minutes on low, then adding water and other ingredients will help prevent this. But diffuser will get discolored even without the water coming through.