I have never experienced this myself, it is just something I have read on the cast boolit forum and in the instructions I read on alloying metals. Guess I should have tested it before I stated it.
I did understand that it wasn't an issue with round balls, just boolits with lube grooves.
It depends on the mold you are using, because of the different materials used to make the molds.
Alum. molds heat up faster.
Steel molds heat up slower.
This is one of the things I learned from experience -- casting small jigs, I'd always have voids, then I learned to let the mold get good and hot.