Craft multisite site without www redirects unwanted to default site
The problem is that the force ww redirect isn't setup correctly. This must be setup like this:

    # Force www
    # -----------------------
    RewriteCond %{HTTP_HOST} ^domain-a.com [NC]
    RewriteRule ^(.*)$ https://www.domain-a.com/$1 [L,R=301,NC]

    RewriteCond %{HTTP_HOST} ^domain-b.com [NC]
    RewriteRule ^(.*)$ https://www.domain-b.com/$1 [L,R=301,NC]


Also both domains must point to the same server.