This is not a regression. Tested in //UE4/Release-4.27 CL18319896
Retainer Box does not propagate Visibility settings to children. Setting Canvas visibility to 'Not Hit-Testable (Self & All Children)' also does not change children of a Retainer Box.
The Button doesn't change to the hovered state, and isn't clickable.
The Button changes to the hovered state, and is clickable. Any logic fired from an On Click event will be executed.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-161583 in the post.