Hi Simon. I've seen this issue from time to time as well and every time it has been an issue with component children, never with the parent device. I just had the same issue now with a device I just shared from a C5 to a C4. After rebooting the source hub (the C4) the component children came back online on the C5. I don't think the hardware matters as I've seen this on all hubs from C4 to C8, and I know I've seen it since at least version 2.3.4 (today's instance is happening on 2.3.8.139)
@bobbyD suggested this could be caused by both hubs being restarted at the same time, but in my case none of the hubs had been shut down or rebooted after I started sharing the device over Hub Mesh so it looks like it potentially was the initial sharing of the device that failed to bring the children online, or they went offline a short time after having been shared. However, not too confident that the children will stay online. Only time will tell 
Are your children still intermittently going offline?
