I am not sure when it started to happen, but IMO after upgrading to 6.46.x firmware. I have tried both hAP ac2 and brand new (bought today) RB4011iGS+5HacQ2HnD-IN.
Let's focus on hAP ac2 as first.
When it starts up, the 5 GHz starts broadcasting SSID, devices are connecting to it, and then it disappears for a while (20-30 secs) and comes back. Then it has problems with keeping up the signal. It doesn't matter if it's under CAPsMAN or normal (out-of-the-box config).
RB4011 does similar, however, it takes much, much, much longer for 5 GHz radio to start broadcasting SSID. When it's under CAPsMAN it is a bit longer, however in the log you can read that it is started up, but no information appears in Wireless section. After 1,5-2 minutes it starts broadcasting and works until it's turned off and on (you don't have to reboot router to repeat the situation, if you turn radio down and up it is the same). Is this normal? I have checked 6.46.2 and 6.47 beta - the same.