In the periodic regionServerReport from RS to master, we will call master.getAssignmentManager().reportOnlineRegions() to make sure the RS has a same state with Master. If RS holds a region which master think should be on another RS, the Master will kill the RS.
But, the regionServerReport could be lagging(due to network or something), which can't represent the current state of RegionServer. Besides, we will call reportRegionStateTransition and try forever until it successfully reported to master when online a region. We can count on reportRegionStateTransition calls.
I have encountered cases that the regions are closed on the RS and reportRegionStateTransition to master successfully. But later, a lagging regionServerReport tells the master the region is online on the RS(Which is not at the moment, this call may generated some time ago and delayed by network somehow), the the master think the region should be on another RS, and kill the RS, which should not be.