Zigbee graph, more devices then registered

Having some issue where every few months, I lose between five and ten devices from my zigbee network. It's random what devices, only similarity is it's always battery operated devices.
I tried debugging what devices do not play nice, and thought about checking the zigbee graph functionality.

Strange thing: found 77 of the 75 registered devices?!

Also, when looking at the "last message" tab in zigbee details, some devices show "0" messages. However, they do respond, and are visible on the graph..