After update to 4.17 button override event - OnMouseButtonDown not working, and because of that drag and drop functionality not working.
Hello Killedge,
This issue appears to be similar to a known issue. I believe they have the same underlying cause. I have provided a link below to the public tracker. Please feel free to use the link below for future updates. If you believe that your issue is separate from the one mentioned please provide additional information on how to reproduce the issue in a clean project.
Link: Unreal Engine Issues and Bug Tracker (UE-49100)
Make it a great day
4.18 Still not working?
Is working in 4.18 but it need a little tweaking - use override on mouse button down preview instead the normal on mouse button down.