Welcome to the forum @IanLauder.
There should be no difference between a reboot startup and restart. I wonder whether node-red is starting up too quickly before the network is ready, or something like that. Even then I would have expected the nodes to reconnect automatically.
Can you post the startup log you get when you stop and start node red manually, then reboot and when it is up, before doing anything else, run, in a terminal, node-red-log and post that too.
Copy/paste the logs rather than a screendump please. See this post for details of how to post code and flows so the forum does not mess them about - How to share code or flow json
