When there is a Retainer box with the visibility set to "Self Hit Test Invisible" and has children, the children cannot be interacted with. Children such as a button cannot be clicked. This issue can also affect children of the child of the retainer box (such as buttons in a scroll box if the scroll box is a child of the retainer box).
CL Tested in:
4.14.3 - 3249277
(Not occurring) (Issue occurs)
4.15.3 - 3450819 (Issue occurs)
4.16.1 - 3466753 (Issue occurs)
This issue does not occur in 4.14.3 binary build of the engine
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-46202 in the post.