I apologise for the late response on this. Currently, when a Mortgage is selected in a Discharge of Mortgage instrument, it looks for all the title in the workspace has that Dealing Number and applies it to the discharge.
We willreview this for a potential future enhancement to the Discharge, however there is a workaround that may work for you right now:
* Only add CT1 (the one with M1) in the workspace.
* Create a Discharge of Mortgage for M1 (the Discharge will only list CT1)