Not able to add an artifact link when the fields permissions on artifact are mandatory. To reproduce the issue you need to follow these steps.
1. Set an hierarchy between 2 tracker (parent-child)
2. Make the Field "Permissions on artifact" mandatory in the 2 trackers.
3. Create parent and child artifacts making sure that you set a value in the artifact permission fields.
4. Add an artifact link between an artifact from the child tracker to the parent tracker.
This is the error :
The field Permissions on artifact (permissions_on_artifact) is required.
Fields are not valid