Starting with the premises that spring constant $k$ and displacement, and therefore the elastic energy stored in a spring are frame invariant quantities, here's a thought experiment leading to the question.
A compressed spring containing Q joules of elastic energy rests on a frictionless horizontal surface with its axis parallel to the surface and one end fixed to the surface. A block of material is placed in contact with the other end. The spring is released, and a stationary observer correctly concludes that since Q joules of elastic energy have been transferred to the block, the block now has Q joules of KE.
A second observer, moving in the same direction as the block with velocity $v$ which happens to be equal to the final velocity of the block in the stationary frame, agrees that Q joules of energy were transferred to the block from the spring as kinetic energy, but observes that the result was that the block stopped and has zero KE.
That only works mathematically if the second observer calculated the initial KE as negative. But KE is never actually negative, so it appears that the mathematics associated with frame dependence of KE does not describe reality when a frame invariant, non-KE form of energy is transferred to an object as KE.
How can this apparent contradiction between energy conservation and frame dependence of KE be explained?