This problem is caused by enabling the option 'only show records for logged-in user'. It happens in 2 cases:
The portal is embedded.
Inline editing is enabled.
We recommend disabling inline editing of the linked records. This is a known issue in our current implementation of the filter function. We realize that it's currently broken in some cases. We plan to rebuild this function from scratch in the future. The goal is to make it consistent whether the portal is embedded or accessed directly. And whether the editing is happening in the form or inline.