Here it is: https://gitlab.com/project-springtime/project-springtime.gitlab.io
The problem occurs with content/systems/_index.md, as soon as its child content/systems/vert-rdwc-1/_index.md gets renamed to content/systems/vert-rdwc-1/index.md. When I do that rename, the parent (systems) shows all .md pages recursively down. While currently those are on the same level as the child index.md (e.g. content/systems/vert-rdwc-1/construction.md), moving them into their separate directory (e.g. content/systems/vert-rdwc-1/construction/index.md) makes no difference.
The documentation seems to imply to me that the parent (content/systems/_index.md) should only show the single child, not all descendants recursively.
And ideally I’d like to decide which one I want. But even if I want recursive, I probably don’t want them as a flat list.