Nice, mystery solved!
And I guess TTN supports LoRaWAN 1.0.2 then, which still says:
2.5.4 AU915-928 JoinAccept CFList
The AU915-928 LoRaWAN does not support the use of the optional CFlist appended to the JoinAccept message. If the CFlist is not empty it is ignored by the end-device.
Whereas 1.0.3 states it’s actually supported:
2.6.4 AU915-928 JoinAccept CFList
The AU915-928 LoRaWAN supports the use of the optional CFlist appended to the JoinResp message. If the CFlist is not empty then the CFListType field SHALL contain the value one (0x01) to indicate the CFList contains a series of ChMask fields. The ChMask fields are interpreted as […]