HI @Anonymous User,
I am not expecting the Teaser Text to show up in the blog's content.
I am, however, expecting the Teaser Text to appear anywhere a snippet of the blog is surfaced.
For example, as you and @DanPatterson have pointed out, the Teaser Text is correctly shown for Latest Activity on the Blog Board.
In other locations where the same kind of card or snippet view is presented for Latest Activity purposes, however, such as Latest Activity under the Community view (https://community.esri.com/) or Latest Activity in my own profile, then the Teaser text is not being displayed. Instead the top of the blog content is being shown.
I would expect it to behave the same way in all the places you are showing an excerpt for Latest Activity purposes?