Throttle cutting out when flying in guided mode?

I’m trying to switch into guided mode in the air (with an onboard computer controlling some surfaces) while maintaining pilot control of throttle. I’ve gotten it to work on the ground with FS_GCS_ENABLE=0 and THR_SUPP_MAN=1. However, when I try to switch over in the air, throttle immediately cuts out. Any ideas why? I’m using a Pixhawk 2.1 Blue Cube.