This issue was fixed in Mass Merge version 1.1.0.31 and can be resolved by upgrading to that version. If you upgraded to Mass Merge 1.1.0.31 and still experiencing the issue, please contact support. 

If your RE application (legacy or NXT) is hosted by Blackbaud, you can expect this upgrade to be automatic. It is currently which is currently scheduled to be deployed to all hosted environments in the first week of November 2018.

Other clients may download the latest version from the Blackbaud.com website when it becomes available.