Answer
A page's apparent LU in IEE isn't necessarily the actual Logical Unit backing its Aurena projection's underlying view — trace the specific view a given Aurena page/list actually reads from via the Debug Console to find its real backing LU, which can sometimes differ from what IEE shows, and can even change from one Update level to the next. Separately, note that a Logical Unit without its own base table, a view-only/joined LU rather than one backed by a real physical table, generally only supports ReadOnly custom fields rather than full read/write ones — if the page's real backing LU turns out to be this kind of table-less LU, that's an inherent limitation of extending it, rather than a bug or missing permission.