Neighbor reports/802.11k missing on 5GHz?

[beetlrokr]

I'm running two sites with Unleashed 200.14.6.1.203. Site1: 4 hardwired AP's, 4 Wi-fi Networks; Site2: 3 hardwired and 1 mesh, 1 Wi-fi Network.

In both sites, background scanning is enabled in System->Services->Radio Control for both bands, and both Background Scanning and 802.11k Neighbor-list Report enabled for all Wi-Fi networks. All AP's have both 2.4 and 5GHz enabled. 

I've tried changing the Background Scanning interval to various values from 60s to 3600s.

The problem: I don't see the expected neighbor 5GHz channels in the Access Point->Show AP Info->Neighbor AP's list. Here's what I see:

Site 1: each AP has all neighbor AP's 2.4GHz channels listed (signal strength values from 9-33dBm) but no 5GHz channels

Site 2: the mesh-root AP and the mesh-child AP have only the other mesh node listed as a neighbor (on channel 108), and no 2.4GHz channels. The other two AP's (hardwired) show the three other 2.4GHz channels as neighbors and no 5GHz channels.

The 5GHz tx power level is "full" on every AP. The 2.4GHz power is -9dB on every AP.

Is there anything I can do to get 5GHz channel neighbors on the AP's?