Is this possibly an issue with your knowledge base / portal setup and the link then presented by Now Assist forces the portal view? Take a look at KB0860116 and see if it helps. The ability to view a knowledge article (including via links presented via Now Assist) should inherit the permissions from the knowledge base unless those are explicitly set at the knowledge article level. If set at the article level, it will override what is set at the base level. 

https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0860116