By clicking a red link in https://el.wikipedia.org/wiki/Πρότυπο:Article_templates/lists/Σπήλαιο once logged in, a page gets created in the user's sandbox space for editing. Once published, the page cannot be edited by VisualEditor as the content seems as empty there. User has to start editing in Code Editor and then switch to VE to get the content in VE. Even if shown once, the content can be lost again on the next try to edit the page.
(There is also a secondary phenomenon affecting the appearance of the template on top of the same page, which states that the page is a sandbox. The template renders differently in VisualEditor during the first edit of the page, than after the page has been published. After publishing, the template renders in VE as if the page is being viewed, not edited. This did not happen in the past, when the template was not cached inside VE. Although this does not directly relate to losing the content of the page as described above, it might be resolved along with the lost content caching issue by using the same approach for solving both: by disabling user sandbox subpage's caching when {{REVISIONID}} magic word is met in the page.)
Editing in user sandbox subpages is used for education programs and editathons by new editors.