In our mortgage with FA, they tell us the monthly figure on the covering letter, e.g. €1000, and the amount on the statement is always that figure less the TRS credit. E.G. if the TRS is €100, then the amount transferred into the mortgage from the current account is €900.
So it looks like the total figure transferred is the sum of the TRS credit plus the net figure transferred from the current account. Therefore the net figure would change if the TRS credit changes.
Look at your statements and see if the monthly repayments including tax relief have remained constant.
Hope this helps
Buddyboy