I have also had random nightmare issues with my zigbee network but narrowed it down to interference. Moved my 2.4GHz WiFi channels to one end of the spectrum and the zigbee down to the other end (including Philips Hue). All necessary due to installing a Wiser heating system that also uses a private zigbee setup (but has no method for changing channel).
I also reduced the power of the 2.4GHz WiFi to the minimum to keep WiFi IOT devices online.
We are fortunate that we live in a relatively unpopulated area with minor WiFi interference from neighbours but I can imagine in more densely populated situations, a neighbour could have installed something new that forced your own WiFi to move to different channels that are then causing problems with your zigbee signals.