update: I found out JOIN to be breaking factor in current flow, as join basically waits 2 or 3 messages till it passes to Function node, which in real world means to press 2 or 3 different switches to actually invoke their sync.
So i've thrown away join node, and inside function node I no longer cares about which topic is the trigger as I (and tasmota) can live with double cmnd to trigger node, eg. tasmota ignores that command if switch is already in requested state.
Now it does look like working reliable.
What is now on the table are some corner cases in case wifi/reboot etc.
