Generally for the Pixhawk the throttle channel should be reversed on the transmitter.(Channel 3) for Futaba. Please insure this.
We use that receiver and transmitter exclusively and that could be your issue.
Though not related, know that the R7008SB is a “Transceiver” it sends telemetry back to the transmitter and as such draws more power then a non-telemetry receiver. it may overwhelm a standard 3DR power type dongle causing low voltage to the FMU. check and power accordingly.