@blues_clues I am thinking there must be some sort radar signal in your area using the DFS frequencies.
As you may know, In Canada DFS channels are reserved for radar use. The related frequencies are however permitted for use so long as a radar detection mechanism is in place in the hardware. This means, while in use, If a radar signal is detected the gateway must change from the DFS channel for a predetermined period of time. Once that time period has expired the gateway will attempt to switch back to the DFS channel configured in the gateway so long as the radar signal is not present.
A channel change of this manner does not change the configured channel in the GUI, only the channel which is being broadcast on. If the radar signal is always present and in range of the gateway you will not be able to use the DFS channels although you can select them in the GUI with the feature enabled.
I did re-test your described scenario to ensure DFS channels were working. My gateway remained on the configured channels (132-136-140-144) and continued to broadcast in that range through a power cycle.
Did it ever switch back to the configured channel? Mine never did, even after several days. Next time it does it, I will reboot the modem and see if it switches again. It should switch right away if the radar signal is still present.