StaticCatalogVocabulary with AjaxSelectFieldWidget shows only UID on edit page

[ewohnlich]

This error also involves plone.app.relationfield, please advise if I should file an issue there as well or in lieu of this one.

When using StaticCatalogVocabulary with a RelationList/Choice and the StaticCatalogVocabulary, "add" forms work as expected but "edit" forms do not, when there is an existing value assigned to the context's field.

As a proof of concept I edited plone.app.relationfield.behavior to use the example from the docs https://6.docs.plone.org/backend/relations.html?highlight=staticcatalogvocabulary#using-different-widgets-for-relations almost verbatim.

from plone.app.z3cform.widget import RelatedItemsFieldWidget
from plone.autoform import directives as form
from plone.app.z3cform.widgets.select import AjaxSelectFieldWidget
from plone.autoform.interfaces import IFormFieldProvider
from plone.base import PloneMessageFactory as _
from plone.supermodel import model
from plone.supermodel.directives import fieldset
from z3c.relationfield.schema import RelationChoice
from z3c.relationfield.schema import RelationList
from zope.interface import provider
from plone.app.vocabularies.catalog import StaticCatalogVocabulary


@provider(IFormFieldProvider)
class IRelatedItems(model.Schema):
    """Behavior interface to make a Dexterity type support related items."""

    relatedItems = RelationList(
        title=_("label_related_items", default="Related Items"),
        default=[],
        value_type=RelationChoice(
            title="Related",
            vocabulary=StaticCatalogVocabulary(
                {
                    "review_state": "published",
                },
                title_template="{brain.Type}: {brain.Title} at {path}",  # Custom item rendering!
            )
            #vocabulary="plone.app.vocabularies.Catalog"
        ),
        required=False,
    )
    form.widget(
        "relatedItems",
        AjaxSelectFieldWidget,
        pattern_options={  # Some options for Select2
            "ajax": {"quietMillis": 500},  # Wait 500ms after typing to make query
        },

    )
    # form.widget(
    #     "relatedItems",
    #     RelatedItemsFieldWidget,
    #     vocabulary="plone.app.vocabularies.Catalog",
    #     pattern_options={
    #         "recentlyUsed": True  # Just turn on. Config in plone.app.widgets.
    #     },
    # )

    fieldset("categorization", label=_("Categorization"), fields=["relatedItems"])

To reproduce:

1. Create a Page (page1) and publish
2. Create another Page (page2) and in the related items field select page1. Save.
3. Edit page2