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So this could be a "works as designed" situation, but I think solving this would be an improvement to the way that DS Extras finds field templates/functions. I think there this is the same issue mentioned at.
I'm using DS Extras to specify a default field template and I'm choosing "Minimal". So all my Display Suite fields are going to use the Minimal field template. For one specific field, field_event_dates, I choose "Drupal Default" as the field template in the Manage Display UI.
When ds_extras_preprocess_field() gets called, I would think that the theme_hook_suggestions should end up like:
field__date field__field_event_dates field__event field__field_event_dates__event
These are the "Drupal default" suggestions before ds_extras_preprocess_field() is called.
Instead, this field ends up switching to the minimal field templates, as specified in the ft-default variable (via the DS Extras configuration UI).
I think the correct way that ds_extras_preprocess_field() should handle this situation is to just leave the theme_hook_suggestions alone since I am specifying "Drupal default" as the field template rather than specifying "The configured DS Extras default field template". To put it another way, if you select a default field template (Minimal, Full Reset), there is no other way of getting back to the Drupal default field template for a specific field.