Just my actual state:
I provided to my TACman mirror of traffic from APs switch port during the failure. You can see there, that the AP is doing again and again arp requests for MAC address of its own gateway and is getting always response, so on the GW, there is correct record in ARP table. In the same time when I tried to PING from Gateway, you can see, that the ICMP request is delivered to AP but not answered by AP. On the GW, there is no nat translation for this AP, so the AP is not communicating outside from LAN. This behavior is till I shut/noshut AP's switch port (AP is powered externally by power adapter, so the power for AP is still there). Whole time during failure state, the AP is not serving clients on another VLAN (SSID) and the status LED is solid BLUE.
I think, that it is failure of AP not failure of dashboard (Meraki cloud controller) because AP should work also in case, when dashboard is unreachable.
Unfortunately my TACman is quite newbie or looser, because everything which was done by him is this statement: "During any type of fault AP will behave in unpredictable way" .
I sent him this link and link to another forum, where this problem is discussed, but you can take a guess what happened..
He asked me if I have still some APs in failure state (the last one I had was the one I used to capture the traffic during failure and during reconnect) and then He closed my case..
Really useful and valuable support provided by Meraki 😞 , I hope this is only my TACman's person mistake..