Thank you!
Yes, I should be aware.
What seems a bit strange is that, when the problem I was mentioning occurs (and it seems to occur, sometimes, when I try to change channel after a DFS channel was set), there's nothing that works rather than a reboot.
If I try to select another non-DFS (or DFS) channel, change txpower, do wifi down - wifi up... nothing happens (other than the generation of wlan0: link is not ready messages on dmesg), I'm just out of the wireless network and I can't generate an access point with hostapd anymore.
When I reboot, the DFS procedure probably restarts, as you pointed me, and everything is fine.
This issue, by the way, seems to be related to this one.