Wrought sprngs often break because they are just too hard.
Cast springs I might SPECULATE hardness and grain size both play a part.
Good Ol' way of tempering by burning off oil in a dish may work for oil quenched plain carbon steel, e.g. 1095. It is not adequate for an alloyed spring, such as O1. Foundries prefer to cast alloy steel rather than plain carbon, the alloy steel casts better.
Grain size. Very important. If you see the thing look "crystallized" when it breaks, those grains were far too large. Investment castings as they come from the mold are most coarse grained. They need some manner of heat treatment, anneal usually, to refine the grain. BEFORE they are hardened and tempered.
I recall an industrial experiment with investment cast power tool part, 4340 I think, that broke with the most enormous grains. Annealing the @!*% thing first might have made it work, I dunno, was not the boss.
Casting quality no doubt enters into it, but I have no specific knowledge there.
Grain size, grain size, grain size.
Fine grains are very important for best toughness in steel, any kind of steel. Neat thing about steel is that one may refine the grains by heat treatment.
As-cast grains are huge, and there are some @!*% coarse grains in any forging.