Hello,
can you describe how the Futaba receiver is powered?
The problem could occur, if receiver is powered later than the unit. Because during powerup the unit is checking presence of the FrSky cable.
Some modification could cause potential issue, since we have not tested it in other configurations. Was the cable modified in some way? Are cables longer now?
Here we have latest modification of the Futaba SBUS2 including change from ROXXY ESC to KONTRONIK (slot 8) sensor in the transmitter.
http://spirit-system.com/dl/fw/spirit-fw-2.4.6.4dfMaybe it could also improve the situation.