I had this same issue using a slightly different setup:
- FS-iA10B receiver
- FlySky FS-i6X transmitter
- PX4 Pro 1.14.3 firmware
- Pixhawk 6C Mini
I was able to find the issue deep in my RC controller settings:
Menu > System setup > RX setup > Output mode > Serial > S.BUS
This was i-BUS but needed to be changed to S.BUS.
Hope this helps!