There are two lists: A (parent) and B (child). List A has selective access turned on and one of the users is not able to load items into the list Bwithout a parenteven with the proper access to the parent list. They are able to load only with Workspace Admin access. Attached is the error that the user received.
This is because the action creating the child is an orphan without a valid parent. The action in question is creating an orphan like C5 below which only WSA have the permission to do
Make sure the action being used to create the child checks that the parent is valid before it updates the child list. Use formula like FINDITEM for this. Or first create the parent in the same process that creates the child