PINE64

sudo pacman -Syu to update. After update pin no longer works. Permanently logged out. Guess I'll need to reflash...

Make sure you carefully read the terminal screen to check whether there were any errors during update.
In case you didn't reboot yet, you might want to try that first before reflashing.
There is a known bug in Phosh that will redirect you to the login screen right after logging in using the correct pin.
In that case, hold the power button for 20 seconds, and then boot up again.

I had the same problem, a reboot solved it for me.