First off it means false is not set into a return node, In functions it must allways retunr a value, even if false, can’t have a statement without a return. So add another return that has Unhandled as value.
Connect false to second return.
Add Unhandled, as opposed to Handled, as value for second Return.
Then if still the issue persists I’ll try to help some more.
After taking a look at the engine version and running few tests I was able to reproduce this issue on our end. I have written up a report ( UE-26770) and I have submitted it to the developers for further consideration. I will provide updates with any pertinent information as it becomes available. As a note, this issue does not appear to be present in the 4.10 version of the engine if it is viable for you to use the latest release instead of the preview. I hope that this information helps. Thank you for your time and information.