Are you changing it in the Qbt WEBUI settings or the container?
Edit: I found I had the same issue as you, but was able to figure it out. With the base settings, any time you change the port in the QBT interface, it will reset to 8080 when rebooting the docker.
To fix this, you need 2 changes from the default config.
Remove the original 8080:8080 port map
create a new one 6666:6666 (whatever your new port is)
add a new variable
Key: WEBUI_PORT
Value: 6666 (again, whatever your new port is)
apply this and your UI should now work on the new port.
However, clicking WEBUI on your docker in the UnRaid UI will yield a webpage opening with :8080 which won't load. I'm still trying to figure out how to change the default webpage since it seems hardcoded to 8080.
Hopefully this helps you or anyone else trying to change their port!
edit: I figured out that I had the view on basic. Switched it to advanced, and I can change the default launch URL as well.
That should fix all the issues for everyone.
Woot!