Nope, Sage is not adding the hyperlink outside your class. Take a look at the actual mark up that’s being generated. Look here, the_excerpt() is not wrapped in a p tag because it generates it’s own p tags.
I’m willing to bet that in your markup you have <p class="list-group-item-text"><p>, which is not allowed in HTML so your browser is closing your first paragraph tag since you can’t have a paragraph in a paragraph.
Do you mean the mark-up posted above? You’re right that the second <p> is being generated by the_excerpt(). Ideally I’d like it to be in the <p class="list-group-item-text">. Can you think of a way to prevent it from generating it’s own paragraph tag?