If the switch is anything like the one found on the Argon One case, there is a separate script that needs to be installed on the RPi to make it fully functional.
That same script also makes an integral fan function on the Argon case, so I elected not to install it as I don't need the fan at all, however I found that the power switch did function in one direction only without the script, it will turn the unit ON, but it will not power it OFF.
That works for me as I typically leave these RPi endpoints up and running 24/7, so lack of turn OFF is not really relevant for me, however in instances of any software update/reboot, or power outage, I can turn the unit ON by hitting the power button.
I'm curious if your power button might work the same way, and also curious where you found that case, can you post a link?
EDIT:I found the case listing, and I see the few lines of code needed for the config.txt file too. Very cool case, gotta be the ne plus ultra of 3B+ cases, and not just a good looker, total dissipation of heat too!
Do you have a DAC or something else powered by the Shanti's other 1 amp rail? I'm running a Shanti in my bedroom system, one rail powers a 4B, and the other a humble Topping E30 to great effect.
I think Moode re-writes that config.txt file with any SAVED changes to the settings, so you always want to make all other changes or elections first before adding in those lines, otherwise they get erased when you hit SAVE on any other potential changes.
Moode 7.1 is coming before end of Q1 according to Tim, so you might just want to leave well enough alone for right now and wait for that update, unless you have a spare SD card available and can just swap out your instance of 6.7.1 without wiping it.
Congrats on the Shanti and McPi case, really cool stuff.