I too encountered a broken leaf spring leaf in the past as well. Not from overloading or a single huge bump in the road. It was from double railway tracks - where the repeated consecutive bounce and factory leaf springs didn't agree with one another. Ouch!
Like many folks, I also replaced / upgraded 4 x leaf spring packs (at same time) to their +1 size up - using trailer's GVWR number / 4 (then round up) re-sizing calculations. And when tires (rubber) wears out, I also upgrade their +1 higher rating as well. example: Upgrade from factory 225 to 235 size as well.
Never had a problem afterwards....