I have used 3.6.12 successfully with a Tricopter, with Kakute F7 AIO. SERVO6 mapped to Motor7 as the Kakute does not have SERVO7.
After upgrading to 4.0.0, the tail servo (SERVO6) min/trim/max values reset to 1000-1500-2000, and when I set them back to my own values, the output keeps reverting to 1000, while the new PWM values are not saved.
Downgraded to AC 3.6.12 and the tail servo movement is back to normal.
I guess your using BLHeli? The issue is with having the mask as auto it thinks Motor7 is a motor not a servo. Try Turing off SERVO_BLH_AUTO and manually doing the mask for your three motors.
Thank you, this was indeed the problem. Peculiarly, the output remained PWM although DShot is used for the motors.
I hope this gets fixed soon as people might break their servos when binding occurs with the BLH output values.