The full channel mask is sent in the first downlink.
If the end-device is reusing all channels, perhaps ADR has dropped to lowest datarate and enabled all channels again. The network is not aware of the channel mask change accept for join.
Only part of the channel mask will be set by ADR as only 16 channels can be changed in a single command and only one command is sent to adjust DR or TXP.
The application can send ADR commands on port 0 to end-devices to change channel mask.