I am seeing the same issue with cAP ac2 and hAP ac2, and have an open support case with Mikrotik for ~10 months. We see SSID beacon frames (among other management frames) aren't transmitted for up to 10 seconds - this evidently occurs due to a compatibility issue with certain 5GHz clients (smartphones).
Mikrotik state they have opened a case with Qualcomm for the radio microcontroller firmware, and they need to know what clients (smartphones) reproduce the issue, if you can narrow it down; I already passed them one (Nexus 5X) and have a second sitting here. If you can capture a trace showing the behaviour with another co-channel radio with '/interface wireless sniffer', or stream to an ethernet-connected host to wireshark and send it to
support@mikrotik.com, that would help them/Qualcomm.
Finally, it's worth pinging the AP from a client, so you can see when this occurs eg [1]; with scripting, you can dump the registration table when ping latency is above say 100ms and narrow it down to one of more clients.
-- [1]
$ ping 10.36.0.251 -i 3
PING 10.36.0.251 (10.36.0.251) 56(84) bytes of data.
64 bytes from 10.36.0.251: icmp_seq=1 ttl=64 time=0.817 ms
64 bytes from 10.36.0.251: icmp_seq=2 ttl=64 time=0.864 ms
64 bytes from 10.36.0.251: icmp_seq=3 ttl=64 time=0.816 ms
64 bytes from 10.36.0.251: icmp_seq=4 ttl=64 time=1.30 ms
64 bytes from 10.36.0.251: icmp_seq=5 ttl=64 time=0.858 ms
64 bytes from 10.36.0.251: icmp_seq=6 ttl=64 time=0.848 ms
64 bytes from 10.36.0.251: icmp_seq=7 ttl=64 time=0.891 ms
64 bytes from 10.36.0.251: icmp_seq=8 ttl=64 time=0.871 ms
64 bytes from 10.36.0.251: icmp_seq=9 ttl=64 time=35.5 ms
64 bytes from 10.36.0.251: icmp_seq=10 ttl=64 time=3917 ms
64 bytes from 10.36.0.251: icmp_seq=11 ttl=64 time=916 ms
64 bytes from 10.36.0.251: icmp_seq=12 ttl=64 time=865 ms
64 bytes from 10.36.0.251: icmp_seq=13 ttl=64 time=528 ms
64 bytes from 10.36.0.251: icmp_seq=15 ttl=64 time=1143 ms
64 bytes from 10.36.0.251: icmp_seq=16 ttl=64 time=8744 ms
64 bytes from 10.36.0.251: icmp_seq=17 ttl=64 time=7173 ms
64 bytes from 10.36.0.251: icmp_seq=19 ttl=64 time=1172 ms
64 bytes from 10.36.0.251: icmp_seq=20 ttl=64 time=16849 ms
64 bytes from 10.36.0.251: icmp_seq=21 ttl=64 time=13849 ms
64 bytes from 10.36.0.251: icmp_seq=23 ttl=64 time=7987 ms
64 bytes from 10.36.0.251: icmp_seq=24 ttl=64 time=4987 ms
64 bytes from 10.36.0.251: icmp_seq=25 ttl=64 time=2237 ms
64 bytes from 10.36.0.251: icmp_seq=26 ttl=64 time=133 ms
64 bytes from 10.36.0.251: icmp_seq=27 ttl=64 time=208 ms
64 bytes from 10.36.0.251: icmp_seq=28 ttl=64 time=31.4 ms
64 bytes from 10.36.0.251: icmp_seq=29 ttl=64 time=0.855 ms
64 bytes from 10.36.0.251: icmp_seq=30 ttl=64 time=0.843 ms
ping: recvmsg: No route to host
ping: recvmsg: No route to host
ping: recvmsg: No route to host
64 bytes from 10.36.0.251: icmp_seq=31 ttl=64 time=1731 ms
ping: recvmsg: No route to host
64 bytes from 10.36.0.251: icmp_seq=32 ttl=64 time=80.1 ms
ping: recvmsg: No route to host
ping: recvmsg: No route to host
ping: recvmsg: No route to host
64 bytes from 10.36.0.251: icmp_seq=33 ttl=64 time=2573 ms
ping: recvmsg: No route to host
ping: recvmsg: No route to host
ping: recvmsg: No route to host
64 bytes from 10.36.0.251: icmp_seq=34 ttl=64 time=4768 ms
RB4011 5ghz, the same. suppot file sent to support. my Ticket#2019022722003126, problem since February 27, 2019.
64 bytes from 172.26.0.1: icmp_seq=224 ttl=64 time=7.874 ms
64 bytes from 172.26.0.1: icmp_seq=225 ttl=64 time=3.571 ms
Request timeout for icmp_seq 226
Request timeout for icmp_seq 227
Request timeout for icmp_seq 228
Request timeout for icmp_seq 229
Request timeout for icmp_seq 230
Request timeout for icmp_seq 231
Request timeout for icmp_seq 232
Request timeout for icmp_seq 233
Request timeout for icmp_seq 234
Request timeout for icmp_seq 235