Manjaro keeps "shutting down" untimely

You have no swap.
With 16gb thats probably fine most of the time … but I might suggest adding something or other.
zram is usually my suggestion.
But this is unlikely to be the cause of your problems.

Oh … maybe it is swap.

After you run out of your physical memory the system is expected to lock up.

To see a quick zram suggestion click here

Your BIOS is also very out of date, especially considering some of the relatively newish components.
If my search is correct