I think the issue is in this code in frame.ts:

            if (suffix.pathname !== "/") {
                src.pathname += suffix.pathname;
            }

if urlSuffix contains a path (e.g.: foo/bar/page) then by simply concatenating strings, you wnd up with a double forward slash:

https://www.website.com//foo/bar/page

which will lead to 404.
I guess it should be handled by properly appending path avoiding slashes duplications.