I'm looking for weirdness in the SSN generator, and I don't think this is it. But since you are rounding here, I think you can end up with numbers that include the upper limit, and I think you intend it to be exclusive. Floor would fix, if I'm not mistaken about this.
https://github.com/ihmeuw/vivarium_census_prl_synth_pop/blob/522f920a35202a98b9c3afa5f99fc91ff623162b/src/vivarium_census_prl_synth_pop/utilities.py#L256
I'm looking for weirdness in the SSN generator, and I don't think this is it. But since you are rounding here, I think you can end up with numbers that include the upper limit, and I think you intend it to be exclusive. Floor would fix, if I'm not mistaken about this.