Same problem here.
Up to last week, my tags were working fine, and now, when I click the tag itself in the left menu, even though it makes the number of notes included in parenthesis, it appears empty "no notes found".
Amazingly, lots of people are complaining about this for months to come and Evernote didn't even reply.
That's it for me. Evernote is broken for years and it's getting worse every day and they don't care about their users anymore. I'm canceling my paid subscription and migrating to Notion. Can't take it anymore.