Been that way for a while, over the course of the war -- as things ramped up, we had more shell-manufacturing capacity than filler capacity. We'd been shipping some of the shells we manufactured to be filled in Poland, as I recall, because they had some excess filler capacity.
EDIT: Yeah, went to some other countries as well, schlorped up some of their filler output too:
The Army awarded $1.5 billion in contracts to nine companies in the fall of 2023 to companies in the U.S., Canada, India and Poland to boost global production of 155mm artillery rounds. The contracts included procuring 14.2 million pounds of bulk energetics, consisting of TNT and IMX-104 explosive.