Why mounting overlayfs with root as lowerdir produce an empty /proc in mergedir?

Here's the test case: #!/bin/bash more="$@" saved="$EUID" if [ "$EUID" = "0" ]; then echo "don't run as root" exit fi m="/tmp/mount-point" exec sudo unshare -m /bin/bash </proc, as you can probably guess, is mounted and not empty in the _big fs_.