How accurate is a DC fuse's voltage rating?
The voltage rating (for an AC fuse) is related to the rated breaking current and some assumed frequency.
If the breaking current is higher than rated interrupting rating then the fuse may not open expeditiously (see example below).
If the frequency is much lower than typical mains frequency the voltage may need to be derated or the fuse may not be guaranteed to work at all (DC being an extreme example).
Image from Cooper-Bussmann:

Of course current rating has to be reduced for high altitudes because of the lower convection cooling, that also applies to dielectric strength over 1000m. Some (very few) fuses are hermetic, but the fuse holder may also come into play.
See, for example, IEEE C37.40-1993 which shows a derating of voltage to 0.56 at 20,000 feet (6100m) and of current to 0.90.

Edit:
As a concrete example, compare the Littlefuse 5x10mm fuses on this datasheet:
Voltage rating is 125VAC with 10kA max interrupting rating
Voltage rating is 250VAC with 35A max interrupting rating
If you attempt to interrupt 10kA fault current at 250VAC the fuse will arc from end to end and explode spraying shards of glass everywhere. It has no problem at 125VAC. Same fuse.