Reread my explanation. 🙂 The start page and the default topic are different things so it follows they cannot have the same name. That was true in Classic as well.
The start page can be called whatever you want but index.htm is the most common. Welcome is another. See https://www.ionos.co.uk/help/hosting/html-and-css/permitted-file-names-for-home-pages/
It's OK in RoboHelp to have either but what you cannot do is have the same name for both. What happens then is the start page calls itself so you see the output within the output. You must have a topic as the default topic and it must have a different name. If the topic that you show in your second image is called welcome.htm then rename it. welcome2.htm would be OK.
________________________________________________________
My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.
Use the menu (bottom right) to mark the Best Answer or Highlight particularly useful replies. Found the answer elsewhere? Share it here.