Testing on the ground, with NO assist enabled (Q_ASSIST_SPEED=0)…mode=FBWA…if disarmed, motors not tilted (ie normal plane mode) but as soon as arming occurs, motors tilt to VTOL and begin spinning…this seems wrong…it means its impossible to takeoff in plane mode…
Apparently, unless the AllowFWTakeoff Q_Option bit is set, even though it was intended to allow GCS without VTOL_TAKEOFF to interpret TAKEOFF mission item as a VTOL one instead of plane takeoff, it forces the motors into VTOL position in FBWA (probably other plane modes also) on the ground. My guess until IS_FLYING is obtained or some other condition, it stays in VTOL mode
that option bit has nothing to do with the behaviour…anytime arming occurs the quadplane will assume that its in a VTOL mode…having FBWA mode set will tell the FC that a transition is expected…as the throttle is raised, the motors will gradually tilt toward tilt max at half throttle…without an undercarriage for a rolling takeoff, this probably will result in a crash…
If you want to start in FBWA, I would arm first in manual mode (then the FC is in Plane mode) and then switch to FBWA.I have not tried it myself, but I suspect that it works that way.
Rolf
Yes, Andrew suggested that also…it was more just trying to understand the whys…but I never put manual as a quadplane mode…my instinct to hit it in an unusual situation would likely result in crash with a quadplane 
…so FBWA takes its place on the switch (which I dont use in plane modes usually)…
I have been having a similar issue because I have my mode control all on one switch. Occasionally I’ll accidentally go right from qstab to acro skipping right past FBWA. This immediately shuts down the back motor while the front ones are tilting down still. You can see the results in these two DVR clips: