Easily repeatable with the stable version of Factorio.

1) Press one of the 5 Buttons under the minimap in the map view.

2) Drag the mouse without releasing it somewhere else/from the button away, then release it.

I'm not sure if this is intended but the new feature that

"damaged items stack and show the average health"

is not only showing the average health of the items in the inventory but the items have the average health after replacing them. Pics can show better what I mean: Imgur link