Cannot add new metadata values when onebox field uses a controlled vocabulary

[alanorth]

Describe the bug On current DSpace 7.6.2-SNAPSHOT and 8.0-SNAPSHOT it is not possible to add custom metadata values during submission when a field uses a onebox with a controlled vocabulary. It seems to only happen when the field is marked as being not repeatable, though there are other issues with the autosave that can clear these values too.

Short demonstration:

Screencast from 2024-03-01 15-13-05.webm

To Reproduce Steps to reproduce the behavior:

1. Configure submission-forms.xml with a field configured to use an external vocabulary with a onebox that is not repeatable, for example:

            <row>
                <field>
                    <dc-schema>dc</dc-schema>
                    <dc-element>subject</dc-element>
                    <dc-qualifier></dc-qualifier>
                    <repeatable>false</repeatable>
                    <label>Subject Keywords 2</label>
                    <input-type>onebox</input-type>
                    <hint>Enter appropriate subject keywords or phrases.</hint>
                    <required></required>
                    <vocabulary>srsc</vocabulary>
                </field>
            </row>

2. Enter the minimum bibliographic information, including a custom value in the onebox configured above
3. Save the submission
4. View the metadata for the item in your workspace
5. The custom subject value entered above is missing

[tdonohue]

Needs a volunteer to investigate. Pulling over to the 7.6.x board just to keep it with other bugs