Or maybe don't ride along with the rear brake on!
Probably same disc that KTM have fitted to thousands of bikes over the years and we don't see this issue reported, so unless they have gone and sourced a cheaper piece of chinese shit for the 2021 bike I would guess its just down to abuse by the nobhead riding the bike with the brake on.
A rear disc no matter how much you use it should not do that , nothing to do with the nob head riding more the disc its self . I has a disc glowing red once as I had over adjusted the pedal on my 1200 Multi and once cooled down worked fine for the next 5k before I sold it , more like cost cutting. Ktms claim it was down to heat is ludicrous.