same problem, will post a log tomorrow.
think its the torque by the front motor.
today we’ve uploaded plane 3.9. during ground test it is better. tilted motors have the same pwm level.
at saturday (plane 3.8) we had problem like this - rc out 5 and 7 are front motors in quadplane tilted vtol
For me it looks often like that. my qplane is yawing left when the front motor starts. first transition was not successful (to fast 100% trottle -> to much yaw) next with 50% trottle for a few seconds and than full trottle.
will be super hard to do auto transitions like that. any idea?
what version plane firmware?
well configured vtol plane? mask and frame type? I had problems with the stiffness of the gondolas and their twisting.
upload .log 
log: https://www.dropbox.com/s/xvn6hy6r2e99jsq/yaw_quadplane.bin?dl=1
3.9.0 plane settings are almost default, qplane settings are tuned (manualy) normal qplane, frame is quiet stiff its full carbon.