Twitter card type value saves but is not retrieved on form rendering

The Twitter card type value saves properly and is properly exported to config, but when revisiting the configuration form, the default value (summary) is always displayed.

I believe this is an issue how this field type is handled when setting the value as it is the only field of type "select".

Steps to reproduce:

1. Enable metatag and metatag_twitter_cards modules
2. Visit /admin/config/search/metatag/
3. Edit config for a certain content type
4. Choose a value other than "Summary Card" for Twitter card type under the Twitter Cards fieldset
5. Save config
6. Revisit config for that content type, notice that "Summary" is now the value for Twitter card type

I spent a fair amount of time trying to work out a fix for this but found that I was in a bit over my head on the OOP side.