For auto power-off, make sure you do not have any unsaved changes to a performance. The unit will not power off if it thinks you will lose unsaved (using [STORE]) changes to the keyboard settings.

Double-tapping on "ARP SW" next to "FS Assign" should bring up a screen to enter numbers. Or after touching "ARP SW" - you should be able to move the DATA DIAL to change the value. Or you can tap on the "--" (in black background) which should also bring up an option to enter a numeric value.

First pressing on "ARP SW" to highlight, then using the DATA DIAL is easiest for me since some of the options show human-readable options (such as Play/Stop, Live Set +, Live Set -, Oct Reset).

... some additional information (from the reference manual):

FS Assign (Footswitch Assign Control Number)

Determines the Control Change number generated by using the Footswitch connected to the FOOT
SWITCH [ASSIGNABLE] jack. Even when the instrument receives MIDI message with the same Control
Change Number specified here from the external equipment, the instrument assumes that the message is
generated by using the Footswitch.

Settings: Off, 1 – 95, Arp SW, MS SW, Play/Stop, Live Set+, Live Set-, Oct Reset

Thank you, it worked with the data dial (I didn't try with that before).
I tried again also with auto power off and it worked... so I surely had something unsaved yesterday!

No problem. How the system works makes sense once you know. A few of these items maybe some optimizations could be made. I remember early on thinking my power switch was somehow "snapping back" (powering itself off) before I looked more into the defaults and disabled auto power-off. It may have been nice to see some [UTILITY] menu which would relay what shut off the system last (auto power-off, manual power button, or unexpected shutdown meaning pulling plug/line-power failure). Since changing the setting, I haven't seen any unexpected power offs like before.

Also in this case, maybe if left on "forever" with autopoweroff turned ON - a message at perhaps 2-3x the timeout period could say "I would have powered off ages ago, but unsaved settings are preventing power-off as a safety feature". This would self-document the case you ran into.

Not really a totally necessary project for the coders to work on - but if there's free time and it doesn't displace anything else - might be worth doing something similar to both bring more visibility to this feature and help self-document some of the "corner cases" (meaning anything other than active use of the instrument with it turned on and auto-poweroff turned ON).

Arbitrary priority assignment: on a scale of 1-10 (10 highest) - this (the suggested changes) would be perhaps a 2.