The monster spawner was updated to render its inner faces, and the newly introduced trial spawner behaves the same. However, this applies even to their inventory models. This makes their details less discernible on such a small scale.

This also doesn't match the Bedrock Edition, where the spawner has its inner sides visible but still hides them in its inventory icon. It even shows them when being held, but that may not be doable with the current block model system that Java Edition uses.