GST Dropping off invoices

We have had a number of open invoices that have deleted the GST value off each line sometime after being issued.

These have been generated GST inc, hard copy printed and emailed directly. In all instances values are correct as inc GST.

When resupplied electronically the GST in value has been altered to a GST exclusive value while the sale still has GST inc ticked (desktop interface). This is also the way the sale will now appear on the system. This is occurring to random invoices with no discernible pattern.

Anyone experiencing the same, or have insight to why this might be occurring?