If SPI settings are set to SPI_MODE3 clock signal should idle at HIGH and before doing:
SPI.beginTransaction(LTC6811_SPI_config);
being
LTC6811_SPI_config = SPISettings LTC6811_SPI_config(SPISettings(500000, MSBFIRST, SPI_MODE3));
the clock is idling LOW (after doing SPI.begin();).

The problem of this behaviour is that the first message is ignored by the slave. After doing SPI.beginTransaction(LTC6811_SPI_config); for the first time the clock will start idling at HIGH as expected.