Short version: C8 rebuilds the built-in zigbee graph from scratch each and every time you view it, while on C7 the graph starts from its last known state as expected.
Ever since I migrated from a C7 hub to a C8 hub I have noticed uxepexted behavior with the built in zigbee graph. With older firmware the graph would only show a few devices and then the hub would become slow and unresponisve and would need to be rebooted. With the lastest firmware (220.127.116.11) it still only shows a few devices but now seems stable. If I wait some time the graph starts to build out, and if I wait a really long time it builds out completely. If I use the menu to navigate back to settings and view the graph again, it starts all over again.
On a side note, if my memory is not mistaken, the C7 dropped the Beta tag in its title, while on C8 it says the zigbee graph is Beta.
When i updated to 18.104.22.168 from 22.214.171.124 the issue seems to be resolved. It is not a listed change so I assume just reloading any firmware may fix it to clear some caches or something.
Spoke too soon, back to the problem behavior.
On My C-8 I found the Graf is not very good
Seems on my setup it never fully populates or most times just shows a few things.
But mine never shows routes and never shows all devices.
Weird part the Z-Wave one always works for me
I thought maybe the graph was building as zigbee messages were coming in from devices...but then I noticed it was showing several devices that I decommissioned over a year ago...and the super weird thing is that they were showing that they were actively connected to repeaters that I installed long after those devices were physically removed from the network. So I deleted those no longer present devices hoping it would clear things up. No such luck. Can say at this point I have no confidence that the graph is showing accurate information.