mounted filesystem not listed unless new window opened

[Bump]

Following this tutorial on my computer:
https://www.youtube.com/watch?v=F0b4I89LY5E
at 10:56

When running the mount commands, doing "ls /mnt" shows nothing and can't chroot because there's nothing there. However if I open another terminal window as root too, is lists all the contents inside /mnt without any further command and I can chroot properly:
mount /dev/mapper/ubu-root /mnt
mount /dev/sda1 /mnt/boot
cd /mnt
mount --rbind /dev dev
mount --rbind /sys sys
mount --rbind /run run
mount --bind /proc proc
(all the mount commands work properly in the first terminal, but are not shown with ls and can't chroot)

Tried to reproduce the error inside virtualbox but no luck, only happened to me on the physical computer.