I just experienced the same issue. I'm also running on AOS 6.4.3.7
We had a Plant wide power shutdown last weekend for maintenance and we just realized some of our AP-275 were in inactive state since the network is back.
On some switch with many AP-275 sometime just one is in that state.
We are on Cisco 3750X 15.0(2)SE6
Some shut/no shut on the port seem to fix it but that is strange and unusual.
there is some weird date in the AP logs though :
Reboot Information
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(none found)
------------
Rebootstrap Information
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Date Time Reason (Latest 10)
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1969-12-31 16:01:33 New connection, Changing to LMS (10.75.47.11) [cur_lms_index: 0, event: REDUN_EVENT_TUNNEL_UP, cur_state: REDUN_STATE_TUNNEL_MASTER, function: redun_tunnel_up(5301)]
2017-08-18 18:18:00 PoE power status changed, resetting config