In another forum, found someone reporting the same problem.
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I have a similar situation in my house with more than 50 shellies and many other wifi IoT devices (Tuya, Sonoff, Emylo, etc.), all based on ESP8266.
And I too have an Asus Wifi mesh (5 XT8s).
All shellies have a fixed IP and use CoIoT unicast with Home Assistant.
I don't use the shelly app.
Only shellies sometimes become unavailable in HA and don't even respond to direct pings, they are totally unreachable on the network.
Only a power off/on restores the communication.
Instead all the devices from the other manuacturers have never missed a beat in many months!
I too sometimes see that some shelly's state is reported in HA even if that shelly is unreachable on the network.
This suggest to me that in some way the shelly IP stack is frozen but unicast messages are still sent out.
My opinion is that unfortunately the wifi stack programming in the shelly firmware isn't (yet) on par of the other ESP8266 devices. "