Tell me about your ballast resistor failure.

The Nichrome wire gets brittle from the hot/cold cycles it goes through in operation. An analogy is a fuse that is run close to it's rated value. The wire gets hot & it eventually breaks....it does not blow from a current surge. It fails from metal fatigue.

The Nichrome wire is wound on a former to help support it; cavity is then filled with ceramic paste for further protection against vibration/failure. Why some are filled & some not....
Do not know. QC issue?