Glitch automatically serves projects with Transport Layer Security (TLS). When browsers load a TLS secured page (e.g. URLs beginning with `https`), they treat unsecured pages (e.g. URLS beginning with `http` – notice there's no `s` at the end) as unsafe and refuse to load their content. Wiki knows how to work around this problem when an author is logged-in. But for readers visiting pages of other authors, or for new authors who haven't learned this usability trap, the links to unsecured pages are simply not found instead of being transcluded into the browser tab.
This symptom is not unique to glitch. Any wiki guarded with TLS will exhibit this behavior. As it happens, glitch does also serve the wikis over `http`. Authors can invite readers to visit the unsecured URLs when their content references other unsecured wiki sites.