@tsndr I believe the 5V_EXT_IN pin if you to supply 5V to the board from an external power supply. If you aren't supplying this pin with 5v, it won't be 5v!
I guess the 0.85V comes from the resistor that @alankilian mentions.
I use an external 5v supply for my Duet WiFi and have a relay on the 24V line which is controlled by the PS_ON pin. That means I can cut the power to motors and heaters without shutting down the Duet. I think this is what you want to do?
Alternatively you can use the internal 5v supply and connect a momentary switch in parallel with the relay on the 24v supply. Add M80/81 to your config. You then physically press the button supplying 24v to the board so it boots and activates the relay (so you can release the button). The Duet can then shut down the power but not restart it without you physically pressing the button