The AircraftContainerSDKFS2004.pdf writes in the Auto Pilot section something about "default_bank_mode":
This determines the default
bank mode when the
autopilot logic is turned on. 0
= None, 1 = Wing Level Hold,
2 = Heading Hold (current
heading). If no value is set,
Wing Level Hold will be the
default. Note: Setting the
variable
use_no_default_bank=1 will
set default_bank_mode = 0
perhaps this is the reason?