Motors spin immediately when powering on

This is my initial setup with ArduPlane. All calibrations, including ESC, have been done. All is pretty much well here before my maiden flight except one strange error… When powering on the APM, my motor immediately starts running. Throttle is at zero, mind you. If I unplug the ESC’s power, then ARM the APM and replug the ESC into power, all is fine. Motor stays quiet, and throttle works correctly. I can’t figure out why the motor wants to spin (at about 10% throttle) as soon as I power the plane on. Also, I will add that when I power up and the throttle is immediately running at about 10%, I can control the throttle as if it were armed?! It just won’t ever stop. Stays at at least 10% or so.

Can anyone offer a theory as to what’s happening?

What flight mode is it in when you power up?

Always start in Manual Mode.

It even says DISARMED on Mission Planner.

Do you think its this bug?
Do you have arming enabled? If so does the motor run briefly and then stop once the APM has finished booting?
Thanks, Grant.