Im not sure if this occurs outside of 4.11, havent exactly tested it as such and i dont really think this is a Bug more so an annoying side affect.
If a MaterialBillboards bSizeIsInScreenSpace is true, using the Zoom-On feature (F key) when having an Actor selected will zoom out to maximum zoom distance.
Its quite annoying really haha, i believe that it should make an exception or at least Zoom to the Root component.
I did some testing and some digging, and you can check the ‘Use Attach Parent Bound’ option to correctly focus (F key) on your blueprints root component.
This option can be found by selecting the MaterialBillboard component within the ‘Rendering’ expanded dropdown menu.
Try increasing the bounds scale a little bit to see if that helps. Also, is the Material Billboard a direct child component of the Root, or are there other components within this blueprint?
Have you tested this issue without the custom ‘Distance to Size Curve’ as well, to see if that is having an effect on the disappearing billboard component?