Normal spalling should be expected to occur before, during and up to about 6 months after installation as the stacked stone settles into its new home. For exterior installations weather patterns like freeze-thaw cycles or wet-dry cycles can bring an increased amount of spalling, but the amount of debris being removed from the face of a stacked stone wall should diminish over time.
Sealing a natural stacked stone wall isn’t specifically aimed at reducing the amount of spalling that might be happening in a certain installation, but by the very nature of what a sealer is intended to do will help. A sealer, by reducing the amount of water that penetrates into a stone, reduces the amount of movement in stone layers caused by moisture absorption thus helps reduce spalling.
Spalling that is occurring beyond the normal range of time post installation or happening as a result of repeated triggers, such as a freeze thaw cycle, might be the forerunners of bigger problems in a stacked stone installation. If certain steps were missed, such as the application of a water proof membrane in an exterior installation, or if the wrong type of stone is being used for a specific application, such as a stone that is not freeze-thaw rated / tested being used in a cold climate, there could be bigger problems afoot. Contact your stone machine supplier with information about what is going on and they should be able to help you out.