Adam Drew explains this problem and how to address this in this comment on the CDN re-route ticket for containerized/stage:

Caddy expects that there will be two builds of the app - one preview, and one stable - located in the /opt/app-root/src/dist directory in two subfolders: /opt/app-root/src/dist/stable and /opt/app-root/src/dist/preview. When using the centrally managed build scripts all of this is done automatically. However, because you are providing your own ci.sh you are doing the build yourself and not creating the required disk layout for caddy.

You have a couple of ways to fix this:

1. Get rid of ci.sh. The system will fall back to using our provided build scripts
2. Extend your ci.sh to provide the expected disk layout. Ensure that your app's built assets end up in dist/preview and dist/stable
3. Ship your own Caddyfile to match your own file layout

I recommend option 1 because it is the least invasive, you are removing instead of adding.

Before following these recommendations, it might be worth to first add a definition for stage/stable deployments (on top of the existing stage/beta) at the ocm/ui/deploy.yml config file, and see if the problem still appears.