I definitely have to plan migration to Esphome 
Found 2 of the Tasmotas not available today, even without an HA restart in between.
My automation didn’t work; found out my Tasmotas does not subscribe to the topics to process commands sent via MQTT publish (no Subscribe messages at reboot).
I did only test the command on the console, my bad.
Anyway on the information tab I found out they subscribe to a couple of group topics including the tasmotas one:
This is very convenient for the automation as you only need to publish one command to reboot them all and forget about its maintenance:
On further inspection via MQTT explorer I found out the root cause of the issue, meaning why the Tasmota integration doesn’t work properly.
The integration itself doesn’t have any issue, it’s the Tasmota firmware failing to update properly the LWT (Last Will and Testament) topic.
Both the unavailable Tasmotas were Offline instead of Online.
Seems the integration is relying on the LWT to detect the current status of the Tasmotas.
The LWT should be updated with the correct status once it changes but clearly it doesn’t work as expected.
The status could be set Offline for a multitude of reasons seems, from Wi-Fi failures to MQTT disconnections, etc.
Somewhere in the code it’s not set back to Online when it should (I’m using v14.6.0 and upgrading to v15 now).
Rebooting the Tasmotas clears the LWT (which is retained so it sticks unless updated) with the correct status.