Some users can not see an Object Subpage even though the Subpage is not secure and it doesn't have Display Condition.
Why some users can not see the subpage?
The page does have a display condition, but it is not showed in the UI because the expression contains a Group that doesn't exists.
This is cause by a known defect:
CLRT-79498 - If I have a subpage with condition expression checkGroup(), Clarity let me delete the Group, but the table still have the expression, therefore you can not see the subpage.
Our sustaining team reviewed CLRT-79498and closed it as they are not planning to make any changes to this area of the product.
Workaround: Recreate the Group, that way you will see the display condition in the Subpage.
The only way to confirm if that is the scenario you are experiencing is by querying the table "odf_filter_expressions" and check if the record for that subpage in the field "expression" contains a Group that doesn't exists. To get the query you need to enable the SQL trace, login, open the Object, open the subpage that has the problem, and logout. In the SQL trace file you can identify the query that join the tables odf_views, odf_filter_expressions, view_captions.
If you Recreate the Group you will see the display condition in the Subpage again. Once you see it you can edit it and remove the Group from the condition if you don't need it.
If the the display condition in the Subpage is met then the users should be able to see the subpage.