The data sheet for the resistor used by the original poster is rated up to 275o C ambient temperature at zero dissipation, which corresponds to the maximum operating temperature of the resistor itself.
This is noticeably higher than the junction temperature rating for power semiconductors, typically 200o C for silicon.
However, FR4 PCB substrate material is rated up to only 110o C to stay below the glass-transition temperature of 130o C.
Higher temperature rating substrates are available.
That's why you elevate them above PCB. 100
oC is fine for any PCB material as long as resistor is sufficiently raised and pads are not super tiny. Junction temp for most of silicon semiconductors is not not higher than 150
oC, for some MOSFET it's 175
oC. SiC MOSFET usually go up to 175
oC, but I've read that they can go to about 200
oC,
However, FR4 PCB substrate material is rated up to only 110o C to stay below the glass-transition temperature of 130o C.
It's a very overbroad statement. There are many substrate types which are categorized as FR-4 but have vastly different properties. 170
oC Tg is not rare at all. 130
oC Tg is as low as you can possibly get from what I've seen.