constraints fail on play

Hi all,

I have a blueprint setup that auto constrains a grid of cubes together via a construction script. It was working just fine in 4.3, but not seems to be broken in 4.6 and 4.7 preview.

I can see the constraints just fine in the editor, and links between each cube are clearly being drawn.
Here is a screenshot showing a grid of cubes built via the construction script, and a second blueprint with a pair of cubes where ive set up the constraints manually:


In this second picture in the same objects a fraction of a second after i press simulate in the editor. the link lines and red/blue bounding boxes vanish in the auto constrained blueprint, but the manually built pair of cubes are still constrained just fine:


I have no idea why it is now failing, any suggestions would be great. Cheers.