Invoices are created using a saved FAST batch. When viewing the invoices in Records, the GL Distribution shows the GST amount but the PST amount is missing. In this instance, this happens only when saving and closing the batch, or not committing directly after the batch is created. If the batch is committed immediately, the PST does not disappear.
Download and install the latest patch, which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.
Alternative Solution: Commit the batch before saving and closing the batch.