Your reply had me looking further and I have this:
CLRT-69994 Can not add linking to a custom subpage to the NSQL Query if subpage was created in object partition
The description for it was:
If the object does not have partition I am able to add a linking to custom subpages.
If the object has partition I am not able to add a linking to custom subpages.
No workarounds were available, and there are no plans at this time to fix it. If this is an important option for you, I would recommend creating a support issue and make reference to this defect in the description, and please include your current Clarity version and patch level too, as these may help.