If you went to Unread, then used the Dismiss button at the top-right, expecting to mark all the old topics as read, it turns out it also mutes those topics (update: But only if you click the checkbox inside the confirmation dialog, which makes it mute in addition to marking as read).
I did this myself, and also suggested doing it in another thread without realising.
If you did the same, or followed my bad advice, you can undo it as follows:
Select everything via the = button top-left of the list (Note that it only selects as much as you have srolled into view, so you either need to scroll a lot, or refresh later and repeat the steps.)
Click the spanner on the top-right
Click Change Notification Level and set it to Normal or Watching or whichever you want.
Do a full F5 refresh afterwards, since the page starts showing you latest topics, not muted topics, after making a change.
F5 Refresh the list afterwards to see if anything is still muted, and repeat until fixed.
Thanks to help from the Discourse forum I've learned that the Unread > Dismiss buttom is fine, and will just mark everything as read, as long as you don't touch the checkbox inside it, which also mutes everything.
I think the first time I used it I must've assumed the checkbox was for confirmation, not to make it do even more.