Update: One additional symptom is that when I try to do certain things on my kids' Nest Hubs (like manually trying to control media - not with my voice) the Nest Hub says that the action isn't allowed when Downtime is on. The issue seems to persist when I manually turn off Do Not Disturb. I am pretty sure that the Do Not Disturb issues and this one stem from one of two things:
(1) Firstly, I have the "It's almost bedtime" Family Bell running in my Google Assistant. This Family Bell sounds on all three of my kids' Nest Hubs in their rooms on school nights. In theory, it's supposed to remind them that bedtime is soon.
(2) Secondly, the kids have cheap Android tablets and supervised accounts linked to Google Family Link. The tablets are set to lock at a certain time every night. However, I find it hard to believe that the Nest Hubs are linked to Family Link and associated with my kids' accounts, since at no point in the Nest Hubs setup was I asked to formally associate the Hubs with another particular person's account. The kids are associated with the parents' accounts through Family Link but the kids are NOT members of the Google Home (i.e. they can't add or remove devices, etc). So for example, when David's tablet is locked, how does the Nest Hub in his room know that it "belongs" to him? Is Google smart enough to know that the virtual "room" on Google Home called "David's Room" should be associated with David's supervised account? I doubt it.
I think it is more likely that option (1) is the culprit. It is likely that the Nest Hubs are going into some sort of Downtime mode linked to the Family Bell. I have therefore disabled the Family Bell and I will see if that stops this unwanted behaviour in the Nest Hubs.
Once I know the answer I'll provide an update.
