Source: https://twitter.com/Aviation_Intel/status/1142532802237194240
Tyler Rogoway raises and interesting suggestion, that the "cube in a sphere" things that have been reported are actually some form of radar reflector. Weather balloons and boats have radar reflectors made of three intersecting planes of metal or metal coated material. The corners are "retroreflective" meaning they show up really well on passive radar systems.
He found an old patent, U.S. Patent #2,463,517, which describes a radar reflector inside a balloon. He goes to to speculat about various ways this might be used, possibly that they were launced from a submarine to test the fleet's radar defenses.
Looking through the citations to that patent I came across US3671965A from 1972, with this fun diagram:
This is a radar countermeasure. It's lightweight, collapsible, stored in a tube, and when it is shot out it springs into shape. It's not a balloon, but it would still look like a sphere in a ball.
So is it possible that during training exercises pilots were practicing deploying something similar, and then a few thousand feet lower another jet in the exercise (or even the same jet, they would take many minutes to fall) would see it. Having never seen it in the expanded form, it might look rather odd. The relative motion of the jet would make the reflector look like it was moving fast.