Step counter and heart rate monitor suddenly stopped working

My child has a 165 they used to track running etc and to date it has worked fine.  However, this week end they have been away on a Duke of Edinburgh expedition (for non UK, this is a scheme for teenagers that includes a trek over a couple of days, with backpack, and involving wild camping etc.  Unfortunately, after working for several months, the watch chose these days to suddenly stop counting any steps, distance travelled or heart rate.  The counts for both were 0.  The watch was not rebooted before they went, just charged to max.

Other that that, the watch seems to function fine (e.g. the screen is there, you can scroll etc), it is just that those metrics were not captured. There was no green flashing light on the monitor either. After a reboot a few minutes ago it seems to work fine again but we'll keep an eye on it over the next few days to see if the issue resurfaces. There is obviously a bug or issue at play here and so I was wondering whether it had been raised before (a search found something similar for other watched, but not the 165)?

I have to say that it also a real shame and let down that Garmin's software failed to captured an achievement such as the Duke of Edinburgh trek - looks like you can't trust it to do what it sets out to do when you really need it.