Unfortunately I have not found a fix for this yet. I've been hopeful for a couple years now that maybe the next firmware will fix this, but it never does. If I want to use the 5GHz radio, I've learned to live with the fact that beamforming cannot be used. I've even tried a brand new clean config, but as soon as I turn beamforming on, the 5 radio channel goes to "unknown" and stops broadcasting. I have the same hardware as you, a 1200AC v1.
I once captured the following dmesg output when the radios are reset showing that ath0 (5GHz radio) goes into disabled state and never enters forwarding state like the rest of the interfaces:
[ 184.961790] br0: port 1(eth1) entered learning state
[ 198.725730] br1: port 1(ath0) entered blocking state
[ 198.730725] br1: port 1(ath0) entered disabled state
[ 198.735867] device ath0 entered promiscuous mode
[ 198.781834] device ath0 left promiscuous mode
[ 198.786236] br1: port 1(ath0) entered disabled state
[ 199.392068] br1: port 1(ath1) entered blocking state
[ 199.397071] br1: port 1(ath1) entered disabled state
[ 199.402219] device ath1 entered promiscuous mode
[ 199.630828] br1: port 1(ath1) entered blocking state
[ 199.635833] br1: port 1(ath1) entered forwarding state
[ 199.678530] br2: port 1(ath1.1) entered blocking state
[ 199.684167] br2: port 1(ath1.1) entered disabled state
[ 199.689472] device ath1.1 entered promiscuous mode
[ 199.747034] br2: port 1(ath1.1) entered blocking state
[ 199.752216] br2: port 1(ath1.1) entered forwarding state
[ 200.161807] br0: port 1(eth1) entered forwarding state