function theme_field__content_type() isn't working
The problem lies in having other functions that alter OTHER fields of the SAME content type.

Earlier in my development, I created two functions, one each, for two specific fields. Even though my adjustments for those fields were minor (added classes to one and added an img tag to another) it still caused the generic function to fail.

Because I had those other functions...

theme_field__field_name__content_type


It made the generic function for all fields...

theme_field__content_type


...not work.

Once I removed those other functions? Suddenly theme_field__content_type started working instantly.