As described below it could be a case that e.g. a select field had a value type configured as Long, however, no converter specified. The underlying JCR back-end would handle that correctly (would convert select's internal string value to Long and would store it appropriately). 6.2 fields have a similar logic actually (see JcrItemPropertySet implementation), but we still bail out from attempting to convert such cases in AbstractConfiguredFieldDefinitionConverter:
Can we just let it slide if the values are primitives?
Option group field conversion is failing in dialogs. To see an example, open the pages app and go to the page /travel/contact. Click the main section, scroll down the page to the email field. Edit the field.