When hovering over the tag you can see that the excerpt is enclosed by paragraph tags (just hover over the tags on this question as a test).
Which kind of makes sense as if you look at the tag info you can see it:
However, looking at the tag excerpt history, it's not there:
So where has the
<p>...</p> come from all of a sudden?
Note: this affects all tags in several sites, e.g. here on MSE and on Stack Overflow.