Try running sudo shutdown -h now
and see if it still does the same thing.
If so, try forcing ACPI actions like so and see what happens: https://askubuntu.com/questions/125844/shutdown-does-not-power-off-computer#127022
I know this an ACPI tables issue, but there’s a wide variety of debug steps to figure out which one.
xdg-open is responsible for handling those. You just need to change what it thinks the default might be: https://www.reddit.com/r/linuxquestions/comments/1ha9czj/setting_default_browser_for_opening_links_with/