Looking at this post it has reminded me that hwdef.dat defines RX6/TX6 as having an inverter. That might be what is causing problems with your telemetry. You could confirm this by one off two ways. Either, comment out lines 86 and 87 in hwdef.dat:
PC8 SBUS_INVERT_RX OUTPUT LOW
PC9 SBUS_INVERT_TX OUTPUT LOW
and recompile the code. Or alternatively, you could temporarily move you telemetry to RX1/TX1. If that does just work then it is likely to be the inverter that is causing the issues.