Everytime I reboot (4.6.3, non-vectored VTOL), Servo 3 and 4 change to TiltMotorFront (right/left)

# Bug Report
## Bug report
### Issue details

Please describe the problem

https://discuss.ardupilot.org/t/everytime-i-reboot-servo-3-and-4-change-to-tiltmotorfront-right-left/143041

I change servo 3 and 4 to disabled - but when I reboot, they are set to TiltMotorFront (right/left).

To be more clear, rebooting does NOT change the servo outputs, but power cycling DOES.

**Version**
4.6.3, 4.7.x beta, and tried all the way back to v4.2.x(?) - two different flight controllers.

**Platform**
[ ] All
[ ] AntennaTracker
[ ] Copter
[ X ] Plane
[ ] Rover
[ ] Submarine

**Airframe type**
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_

I’m building a 2-motor, non-vectored tailsitter.

**Hardware type**
_What autopilot hardware was used? (Pixhawk, Cube, Pixracer, Navio2, etc)_

Kakute F7 (non, AIO), and Pixhawk v1.1

**Logs**
_Please provide a link to any relevant logs that show the issue_

Start with the linked discussion and reproduce the behavior by power cycling the Kakute F7 and Pixhawk v1.1 on the reported Plane versions. No source file or test is named; the issue is done when Servo 3 and 4 remain disabled after a power cycle on the affected non-vectored VTOL setup.

Written by the indexing model from the issue text.