I've got an AP that was just upgraded to 13.4.1 and on boot up it starts the Channel Availability check and gets about 5 sec into it and then goes Idle. It just loops in this process. It does not detect a DFS and move on the the alternate or anthing. If I downgrade the FW down to 12.1 it does away and functions normally.
Here is a screenshot of the DFS page on the FW 13.4.1. The log is just full of that entry.