This could be due to weak signal back to the coordinator, not sure if you have strategically placed routers around to help ( i.e. devices that plug into an outlet ) these devices act as repeaters. Also, this could be due to interference. Zigbee uses the 2.4 MHz spectrum, so your wifi router channel and your zigbee channel should be separated as far as possible. I hope this helps you narrow things down.
If your Zigbee Router devices work properly then should be no need to re-pair as the mesh fixes itself.
As mentioned, one common issue is poor reception / interference on the side of the Zigbee coordinator.
Suggest you begin by follow some of these general recommendation before troubleshooting any further:
I would start by adding a long USB extension cable to get the Zigbee Coordinator a bit further away, and of course, make sure that you upgrade to the latest recommended firmware on your Zigbee Coordinator adapter.
As well as buy and add a few known good Zigbee Router devices (like the IKEA Tradfri Signal Repeater).
Another tip is to shield your computer and USB devices or peripherals connected to computers as well as other appliances or cables/wires located close to your Zigbee Coordinator. As already established if you read those other tips, Zigbee radio can be very susceptible to electromagnetic interference (EMI, also called radio-frequency interference or RFI), and one very common source that generates electromagnetic fields is unshielded computers and unshielded peripherals so make sure that you use for example enclosures for external hard drives and single-board-computers that are made of metal instead of plastic. Especially known to have a huge interference impact on wireless devices using radio frequency like Zigbee radios are USB 3.0 devices/peripherals (for more details on that read white paper report on the subject by Intel at https://www.usb.org/sites/default/files/327216.pdf).
moved my Pi4 upstairs away from wifi AP, this put it in the centre of our house upstairs
every light switch has a zigbee module switch behind it, in total around 30
wifi 2,4 is on channel 2
i have bought some more zigbee repeaters to plug in around the house
Im guessing the drop offs is because the mesh system could be repairing finding the best route etc.
Good news is i had an issue with this 1 zigbee module, that for the last 2 days could not get it past identifying device, but finally this morning it was configurable.
its just devices that have been working fine for weeks, just become unavailable