When a user creates a record in the portal, it will be automatically linked to them. They don't need to select themselves from a link records field (nor do you need to show the field on their form).

You do not need to display the user-linked field for the new record to be linked to the logged-in user. If that reverse linked field is included in the child form and the parent user record is already known, it can be prefilled before submission. The relationship is also applied when the record is saved; a blank or absent field in the UI is not proof that linking will fail.

We highly recommend not adding the "user linked field" to the form. Please remove it if you have it added already and it's causing confusion.