Both sides have been using cluster munitions since the start.
The Ukrainians had a large inventory from the USSR days, which they ran thru. Then they got a bunch from the French who pretended to sign the treaty and destroy their stockpile.
Instead they gave their inventory to Ukraine, which has now run thru that lot.
US is giving cluster munitions because they don’t have anything else left. Just like they started giving them DU tank ammunition, because that is all used up as well.
The Ukrainians barely have any DU-capable tanks. They have @30 Leopard IIs, few of which have even been at the front. How then has whomevers stockpile of NATO 120mm been used up?
I suspect you are relating your own fantasies.
And, being Leopards, they probably aren’t even set up to use American DU.