The more I read about the Categories & Tags pages, the more I feel like you should not use them. I actually just wrote a post about that: No Categories & Tags. Why?! It all sounds good at first but I think that it diverts SEO juice and does not really improve your SERPs.
Actually, I see two major problems:
I don't see my users ever going to those tag/category pages—if my readers don't use them, then they are not necessary, right?
When Google sends one of their user to one of those pages, they always bounce. This is a very strong signal to Google that it's crap and thus should not be ranked high anymore.
People are doing the same thing as you: if they want to find something on your website (or in general), they go to Google, not your Categories & Tags pages. Since I've been removing those from my websites in the last few months, I've seen improvement in my impressions and clicks. So I'm sure there was a rather negative effect in having them in the first place.
Now to answer your specific question, I would say that somehow that one category or tag page got a really high rank (maybe a backlink to it?) and thus Google will often re-read that page and thus find your title and index that new content quickly. The page itself is brand new and the rest of your site may not rank as well so Google takes its time to read new pages. That doesn't mean later searches with the same title would not instead return the new page.
Still, if you have not done so yet, I would remove all the Category and Tags and put redirects probably to the home page (wherever you have your blog.) The only reason you'd want to keep your meta data is if you really receive many clicks on them. If you can see such on your website, you may want to think about it a little further (i.e. clicks from posts to your tag or category pages). Chances are, it's just like me, you get one or two of those every quarter. Not useful.
In my post (link above), please make sure to read about all the detrimental reasons why search engines will penalize you for having tags and categories. It was working a while back by increasing your rank, today, not so much (and even in 2015 the wheel was already turned.)