When you preview (or use the visual editor) in flow boards that are in a subpage (for example https://ca.wikipedia.org/wiki/Viquipèdia:La_taverna/Tecnicismes), the links that you have added link to a subpage of the parent page of the flow board. In the example above, a link to [[Wikipedia]] will link to https://ca.wikipedia.org/wiki/Viquipèdia:La_taverna/Wikipedia instead of https://ca.wikipedia.org/wiki/Wikipedia. When you submit the links work correctly though.