This happens with not only enums, but any combo box. Another example is the "Create Event" where you can select a matching function. This is a problem with how this slate widget determines if it should be displayed above or below the combo box.
1- Create an enum, add a lot of possible values (I reproduced with around 50)
2- Create a variable of this enum type in a blueprint, try to set it and see you can choose the value from the list.
3- Move the node to the bottom of the screen and zoom out (about 50%), and see you can't get to select the values down the list.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-89449 in the post.