That’s not correct. I’m glad you are working but AU915 devices definitely have a DR2 starting point that they aren’t able to go below until ‘TxParamSetupReq’ is sent by TTS (or the local stack implementation isn’t fully compliant, not uncommon for this subtle point). You said earlier you’d configured no dwell, which would also do it.
See more background here
This behavior seems to be unique to AU915 and so is easily overlooked.