Why after running podman container run ubuntu the output of podman container ls is empty?

Because the container immediately exits after running the ubuntu image. This is completely normal and expected as containers designed to run a service or a app and exit when they are done running it. To see the container you can run podman ps -a

If you want the container to keep running, you can run a command like sleep 100 which will run for 100 seconds or you can attach to terminal of the container with a command similar: podman container run -it ubuntu /bin/bash