When both DwellTime400ms set to 'false' in Chipstack NS configuration, no 'TxParamSetupReq' sent to device

The setup is one of the four AS923 sub-bands.
Chirpstack NS version 3.16.4
Device is RAK4200.

For region AS923, RAK4200 default to DwellTime400ms=true.

Need to put both uplink and downlink DwellTime400ms to false, so we configured chirpstack ns:
[network_server.band]
name = “AS923”
uplink_dwell_time_400ms = false
downlink_dwell_time_400ms = false

No TxParamSetupReq command sent in the 1st downlink packet:


If we instead set all or any of the two DwellTime400ms to ‘true’, we can see TxParamSetupReq pushed to device, and device channel plan changes accordingly:

My questions:
1, how can we put both uplink and downlink DwellTime400ms settings on RAK4200 to ‘false’?
2, where are the Chirpstack NS DwellTime400ms defaults coming from? Source code shows that it is comparing those defaults with Chirpstack NS configurations (showing at the top) to determine whether or not to send the TxParamSetupReq command