Hello Ed, for investigating. I was able to reproduce this issue on a blank project with the first person example content included. It may be related to the projectile movement / Bounce hit event rather than standard physics impacts.
I have uploaded the test project here for you to try out:
https://www.dropbox.com/s/5k3pxbjayzp9zat/HitTest.zip?dl=0
If you open the level and shoot the projectiles, you will notice a return of either “metal” or “default” upon the ball bouncing, however if you launch the project, you should notice these hit results changing to 'Surfacetype16 and Surfacetype_Default".
edit: Yes, I’m packaging for x64 windows.