Lithium batteries have a discharge curve that is very hard to track by a battery driven device.
There may be argument about this particular graph, but all discharge graphs for Lithium batteries show the same, near vertical end. Battery driven devices sample and report at intervals and may not even see more than one sample before the battery is fully discharged. Or at least enough that the radio can't reach the next device.
Notice the battery stays above 90% capacity til suddenly it's not. Therefore, a value of 80% reported MAY be the last signal we get. I know my Aeon Multisensor6's are this way.. 100% then 80% and then silence for days. Dashboard will show it at 80% forever 
