What causes this resistor to fail - open circuit and no visible damage?
It is my experience (failure analysis) that 1% metal film resistors are not reliable long term with high voltage DC on them. 

I've also seen huge changes in value without them complete going open.

You might try to fit a MOF (metal oxide film- also known as metal glaze) type if you have room. I recommend Vishay (BC Components nee Philips) VR25 resistors for this service if you care about reliability. They're tested at 100VDC, but have a "rating" of 1600VDC. 

 

There's a hint here that moisture (probably plus elements of the lacquer) play a role, so if you want to recreate the failures I'd suggest putting them in your environmental chamber at elevated temperature and 95% RH and see what happens. Obviously, you want to avoid condensing humidity when power is applied  or bad things (tm) could happen.