There’s some kind of visual or ? bug with the inbox showing unread emails when there aren’t any - when clicking on the number to display any ‘unseen’ emails there’s nothing to show:
I did check on my account. I cannot reproduce. For me a numeral in your indicated position is reliable or absent in the case of no unread messages.
I think the way that threads work with snappymail makes it very easy for unread emails to be lost, as this has been the case in the past. I had to select all emails in the inbox and mark them as read, which worked, but not because I could not find the unread email. I looked, and I could not find it at all period. Makes me think it’s a bug.