That's a good question. We had some APs that worked great on 2.6x, but had DFS hits within an hour of going to 3.0 and 3.1. After 3.3 came out we went back to DFS channels and those APs are working fine on 3.4.1. We have some newer APs in a completely different, much more noisy location that we can't get to work in DFS on 3.4.1 or 3.5.1. I didn't think to go back and try 3.3.x.
Can confirm this issue still persisting on 3.5.2 with 2000's and 1000's