Welcome back! Normally if you log in to your Zoom account first, then come to the Community... you're either logged in immediately OR just need to hit the "log in again" button which checks to see that you were logged in on the Zoom portal.
Anyway... what I do see quite often is that someone clicks their Mute/Unmute button at about the same time that the Host clicks the Unmute button on the Participants panel or the user's video box. I've done this!! And then you notice that they're not unmuted, and you realize that maybe you unmuted them at the same time they were unmuting, and one or the other of you re-muted. So you click again... at the same time they're thinking the same thing! I've learned to only unmute an attendee once... and if they have simultaneously unmuted (thereby remuting themselves)... then they are most likely to think "Oh, I didn't hit the Unmute button correctly, let me do it again", and then they'll unmuted. (Don't get into the Groundhog Day scenario of doing it over and over again!)
I also tend to wait and see if the user will unmute themselves, because many will, and then if I see them talking, I'll be ready to unmute them. It's a fine line between being too quick and just quick enough!
So it's impossible to tell exactly what happened, but that's a likely scenario which I see even the most seasoned Hosts doing on occasion. You wait just another couple of seconds, and they're still talking with Mute on, then you reach down and click Unmute just as they discover it for themselves and click it almost immediately. Sometimes you can't win! That's why in general I'll only click it once.
Hopefully that helps!