I suspect this is a constellation of 1) corruption, 2) marketing for the media headlines, and 3) an overt political attempt to pry more money from the US.
Naw, the whole story is pure nonsense.
Nobody has said they've stopped all shipments to Ukraine.
Some have said certain legal authorizations have been used up, but others are still in effect.
I'd be nearly certain that ways will always be found to emergency resupply critical air-defense ammunition.