Pixhawk 6c Channel failing with PPM config

Total noob here taking baby steps toward autonomous rover… thanks in advance for any insight provided.

Use case: tank steering rover / 4 motors, 2 ESC, channels 1&2

Transmitter: Flysky FS-16X Receiver: FS-16X

Pixhawk 6c with M10 GPS and PPM Encoder flashed from MP with rover firmware. No other config done from MP. Just trying to get basic RC through Pixhawk at this point.

Tested config for RC steering direct to model using PWM to each ESC from receiver, all channels/motors work as expected…

When config is changed to put Pixhawk between receiver and model using PPM from I/0_PWM OUT port to PPM Encoder, only motors on Channel 1 ESC works. Channel 2 ESC is non-responsive to RC input.

I feel like I must be missing something simple, any pointers on what to check?