We have been made aware that the functionality to enable RoboCopy.exe to purge or mirror backup synchronization folders is not working correctly in the following releases of Macrium Reflect: v7.2.3811, v7.2.3825 and v7.2.3858. 
We have found the problem and the fix will be available in the next update due shortly.
In the meantime to you can download a small utility called 'fixpaths.exe' using the link below and run this from any folder on your PC to enable your RoboCopy scripts to function:
After executing, your RoboCopy scripts will execute without being blocked by MIG using the rules defined here: MIG RoboCopy Rules
Note: If you edit your xml backup definitions in Reflect then please re-run fixpaths.exe. This is only required until the next update of Reflect after which fixpaths.exe will no-longer be required.