wrote
or if the company regularly flew so low that they didn't put
in any additives such as PRIST to prevent ice in the fuel.

That is what I suspect. In the Boeing jetliners, we didn't use
the additives (too expensive), but we did have fuel heaters on
the main filters to keep them from "waxing" over. A "one minute"
shot of hot bleed air every 30 minutes usually did the trick,
however I have had to reduce the cruise altitude and increase
the mach number to keep the tank boost pumps operating.

Bob Moore