Are you using a custom firmware? The default AT command firmware does not support SF6. Valid SF are from 7 to 12.
SF6 needs a special handling according to the SX1272 datasheet
Spreading Factor 6
SF = 6 Is a special use case for the highest data rate transmission possible with the LoRa modem. To this end several settings must be activated in the SX1272/73 registers when it is in use. These settings are only valid for SF6 and should be set back to their default values for other spreading factors:
Set SpreadingFactor = 6 in RegModemConfig2
The header must be set to Implicit mode.
Set the bit field DetectionOptimize of register RegLoRaDetectOptimize to value “0b101”.
Write 0x0C in the register RegDetectionThreshold.
I will check with the R&D guys, but I am quite sure they did not implement that.
There is as well a discussion here RAK811 unable to set SF6