Changed logic how will boss react in these cases according to your description. Will be applied tomorrow.
Added some custom changes here, but unfortunately most likely issue will still occur sometimes, this is a global problem for creatures who change state from aggressive to passive and other way around. Our core developer is currently improving threat system, there is still a lot of work to be done, but it's in progress. Closing this report and similar ones about aggro issue and we will return to this issue when work on new threat system is complete.