Yes, unfortunately we haven’t implemented that yet. Good news is that is part of our roadmap, so expect it to be added in one of the future releases.
For now the string value enables you to display the enumeration field, and users can even edit it by changing the string, but you can’t yet provide a selector showing all options in the enumeration.
It’s a common scenario, That Mendix while consuming considers some of the of external entity as different data type. Mendix is not smart enough to consume every attribute as they were in external system. Since all of them come either as integer/decimal or string, Mendix considers the enum as string, same goes for Autonumber also, mendix considers AutoNumber as Int.
Hope it helps!