I have archived a bunch of lists that had tags used only on that list. In the tags view they get displayed with their associated count, and when you click the search returns nothing (I assume since the lists are archived).
It would be great in general if tag counts could be calculated only for non-archived lists.
When looking into tag usages on the global search page, you can check/uncheck mark “Skip archived lists”. Probably it is unchecked in your case.
As for “It would be great in general if tag counts could be calculated only for non-archived lists.” - I agree, but this requires adding some setting. I’ll move this request to “Features” category, maybe will implement it one day.