Q:

Tags are not prefixed in the  on the MSE question

We had a brief conversation regarding tags at the <title> tag of our SE questions in this question.
According to our SE tagging system, most tags will automatically be prefixed into the title of the questions unless it's already in the title somewhere. 

Note that the system automatically prefixes the title with the most
  common tag (unless it's already in the title somewhere) to help search
  engines find it more easily.

Ref: https://meta.stackexchange.com/a/130208/245772 last line of this answer
But I didn't find any tag for this question at the <title></title> markup of the HTML page.
Screenshot:

A:

Ref: https://meta.stackexchange.com/a/130208/245772 last line of this
  answer

You have taken that a bit out of context. The first sentence in that answer states:  

Stack Overflow has an extensive tag system...  

It is discussing Stack Overflow specifically, which is a main site, and (as Shadow Wizard states) a tag being automatically prefixed into the title only applies on main sites, not Metas.  
So the reason you did not see this occur in a title here on MSE is because it's a Meta (as per the name - meta.stackexchange.com) ;)

