Resolution so far in case anyone else has this issue.
APT detected my ZWO ASI0120MM-s was dropping out every minute for 5 seconds. Somehow this seems to have cause a problem with my other USB3 cameras on that or other hubs.
Switching the guide camera to a USB2 connection on the MBeat powered hub worked. Then today I put another powered USB3 hub (Targus this time, not MBeat) into play and connected the Guide camera to that - all stable so far!
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