When helping a user with updating their project from 1.21 to the latest, there were some code API changes to the batching classes.
In 1.44.0, pc.Batch.model was removed.
So the following code no longer runs as batch.model is undefined.
this.app.batcher.scene.layers.getLayerById(pc.LAYERID_WORLD).addMeshInstances(batch.model.meshInstances);
The fix for this is to use meshInstance instead like so:
this.app.batcher.scene.layers.getLayerById(pc.LAYERID_WORLD).addMeshInstances([batch.meshInstance]);