It’s rather difficult to manage complex networks of autonomous nodes - (n*(n-1)) meshes so my approach has been not to look too closely at the detail, but trust the individual nodes to “do the right thing” and just help them see enough RF energy from neighbours. RSSI and other measures are a snapshot in time, and can change due to many invisible factors.
There’s several threads on the forum looking at Zigbee issues and this one, so I suggest some reading, and add more mains-powered devices in the area of the hard to physially reach device.