I think the issue is that there are two block quotes in the User Invited to Topic. You can see the template that’s used if you go to your Admin / Customize / Email page and search for “Invite Mailer.”
Possibly the default display of this email should be improved. You can also try customizing the template so that it displays the way you want it to. To remove the block quotes, you would edit the template to remove the > characters from the markdown. To remove the excerpt, edit the template to remove the
%{topic_excerpt} tag.
The email template’s preview window gives you a good idea of what the sent email will look like, but it’s also a good idea to test any changes you make with a throw-away email address.