Somehow, when using the SwitchableFieldDefinition (link from the MTE), the properties are stored incorrect. The LinkModel from the MTE cannot read the correct properties and no link and title are shown.

It seems to be in the CompositeTransformer#getCompositePropertyName() method.
The field definition is i18n, so the propertyName (linkTypeexternal) will be appended by the basePropertyName ('linkType'). This results in the field 'linkTypeexternallinkType'.

See issue MTE-46.

More info from duplicate (and closed) issue.

Due to incomplete implementation of property name generation, info.magnolia.ui.form.field.transformer.composite.SwitchableTransformer now prepends (parent's) fieldname once more resulting in wrong and unexpected property names, also see protected String definePropertyName() and protected String deriveLocaleAwareName(String).

Instead of:

linkType=internal
linkTypeinternal=acc832d1-4302-4812-a13d-b7446d302dfe

it creates

linkTypelinkType=internal
linkTypeinternallinkType=acc832d1-4302-4812-a13d-b7446d302dfe