HAProxy with custom port (5000) backend

Please help me undestand why this is not working.
I created a front end (https://test.acme.com) and my backend (http://10.10.10.10:5000) and no matter what I do it defaults to http://10.10.10.10 which is another container on that machine, and not the one I want to access.

I even tried adding a second frontend with https://test.acme.com:5000 and that didn't work either - how can I make it respect the port I set on the backend?