I started having this issue a week or so ago where Bluetooth would just be disabled on boot, and my mouse wouldn't work until I used the touchpad on the laptop and went and manually enabled Bluetooth. Inconvenient, but not a show stopper.

Finally annoyed me enough today to figure it out, thanks to this post - https://forum.manjaro.org/t/gnome-enable-bluetooth-at-startup/113131 - an easy solution in the end, but why it would be disabled by default seems a little strange. Especially as its a change in behaviour.

TLDR

sudo gedit /etc/bluetooth/main.conf

change

#AutoEnable=false

to

AutoEnable=true

then reboot, sorted!