Simply change your log. levels.
If sensor is in idle state - send log.1 to Domoticz - Last Seen will be updated with each log 1 received.
When motion detected - send log.0 - first log.0 received will trigger your action and change Last Seen time too.
If you want detect, if sensor hags in triggered state, you can measure time interval for log 0 by LUA or DzVents script.
If too long - take some action.
By the way - it is problem MySesnsors part in Domoticz.
When I am sending switch status from ESPeasy by JSON call, Last Seen time is updated with each
received value. ON or OFF -both.