Trouble with static assets in custom addon with ingress

I’m working on a custom addon with a UI exposed via ingress. Hit it with a browser and I get the index page. I have the startup script for the addon container using bashio::addon.ingress_entry to get the base path and tell the app it’s nested below there. It ends up in the <base> tag and a windows.BASE_URL variable as expected.

I see the static assets being loaded and I see the traffic in the access logs for the app. But the responses to the browser appear to not be the CSS or JS content being loaded. Instead, it looks like I’m getting another copy of the main index page.

Feel like I’m missing something obvious.

Wonder where the ingress peeps are…