Since updating to UE4.17, any custom drop down menu will not display the list properly, instead the window will automatically close as you can see here: Screen capture - 1a81f10b889f5a661653bcffc5e6609f - Gyazo
Updating to 4.18 hasn’t fixed this problem and the only workaround is to select what I want from the content browser and use the arrow to apply it, which isn’t ideal.
Pre-4.17 the drop down worked perfectly fine.