Action and option list headers use subheading text style but they arent bold.
Bold font weight:

Regular font weight:

Looks like it's on purpose if the scss calls out font-weight: 400
// src/components/OptionList/OptionList.scss
.Title {
@include text-style-subheading;
padding: spacing(tight) spacing();
border-bottom: border();
font-weight: 400;
color: color('ink', 'lighter');
}
@sarahill can you please confirm?
You're correct, Dan, the lighter font-weight is not consistent with Polaris styles.