Messages "Hide Alerts" and the Bell Icon

This problem is across my iPad, iPhone and MacBook Pro but let's stick to my iPhone 13 Mini, for now. The problem is that I'm not receiving alerts for messages. In Settings>Notifications>Messages, everything is turned on. In Messages, I scrolled to the top of each conversation and clicked on the sender and found that "Hide Alerts" was turned on. I turned it off in each case. Now, several hours later, "Hide Alerts" is again enabled for all conversations. Why—and more importantly, how do I stop this happening before I miss something important?

Also, when I swipe left on a conversation in the sidebar, as well as "Delete", there is a bell icon. In all cases on my phone the bell had a slash through it, which I think means notifications were turned off. However, this icon doesn't correlate with the "Hide Alerts" button: one can be on and the other off and changing one has no effect on the other. So what is the difference between these two settings?

Thanks in advance,

Steve = : ^ )