Values in _headers files aren't applying for custom domain

I’m trying to apply more aggressive cache headers on certain assets with a _headers file. I’ve made sure it’s ending up in the output directory of my built site, and the Pages dashboard picks them up too:

When I view the page with my unique Pages auto-generated URL for a build, they show up correctly:

But I don’t see these headers get applied when I view the page through my custom domain: They instead go back to the less aggressive value:

Any ideas what I’m missing here?