When using default fields in dialogs empty inputs won't be persisted in JCR – removing existing values from fields will actually be removed.

The switchable field (info.magnolia.ui.form.field.SwitchableField) and its transformer (info.magnolia.ui.form.field.transformer.composite.SwitchableTransformer) used for the link field behave differently:

E.g. when setting an internal link and switching to external link afterwards (and also providing a value for both links), both link properties will be persisted in JCR.

An custom transformer would solve this.