Tested this on 6.9 Beta 30 (ish) and it works perfectly. Looks like you've done two things here
The template shows
and the applicable config line shows
<Config Name="WebUI" Target="8080" Default="8080" Mode="tcp" Description="WebUI - Default 8888" Type="Port" Display="al>
Now, the complete line isn't there (you've cut off the end of it), but what you did is that at some point, you edited the app, switched to advanced view, and then edited that URL to be what you've got above.
You can change the port all day long when you edit the Port, but because the actual URL you've changed previously to the duckdns address doesn't include a reference to either the IP or the Port, nothing is ever going to happen.
The appropriate line for WebUI is
Set up like that, the system will properly adjust the webUI in the drop down to reflect the IP address and the Port number you've selected.
or, if you want it to go to a particular reverse proxy, then
Note that the 8080 IS correct in the above lines, regardless of whatever you change the port to be. It refers to the CONTAINER port, and when changing the port mapping you're adjust the host port.