To fix this issue:
1. Open K2 Designer.
2. Find the objects that are "hidden" from the artifacts page of the list.
3. Create a new category folder, for example "Objects Repository".
4. Right click each item that is hidden from the list and use the Move To option to move these items to the newly created category folder.
5. Once the items have been moved, simply right click the objects and Move To the original category folder.
After making these changes the artifacts should again be visible in the artifacts page of the list in question.