Why does the header of the tab shows the first tag and not the title of the question as other stackexchange sites?
That's done to boost the Google ranking of Stack Exchange pages. It's been the case since 2010. Google's ranking algorithms have changed quite a lot, I don't know if there's been any reevaluation of whether this is still relevant.
It happens on all Stack Exchange sites, not just this one. But it doesn't happen for every question: if all the tags of the question are already a word in the title, no tag is added. Also this isn't done on meta, presumably because Google ranking of meta posts isn't important.
If it annoys you, you can remove it via a browser extension. That's what I do.