(I have no experience in adding new control types via coding, apart from using combination control type standard function. My answer below is for a generic custom field created via entity configurations)
Same thing as happened to me in the past where the drop down , in my case a custom enumeration, was not enabled for the field.
Then I removed the field from the form, unpublished the Entity Contribution and published again. When I added it again to the page, it was happily showing me the drop down.
You could also try refreshing Aurena projection and metadata caches to see if that has any effect. Please follow the instructions mentioned in the following post: