When the $MFT file is moved, the mft reserved zone is reallocated after drive dismount/mount. If there is empty space available right after $MFT, windows will make it MFT reserved zone. The size of the zone is between $MFT and the next file or 12.5% of the drive (default), if the free space is large enough.
I would like to suggest adding an Option that limits the reserved zone size by leaving x ~ y MB of freespace after the $MFT file. 10 ~ 20 MB, for example, will only vacate space if the reserved zone drops below 10 MB. Treat the reserved space as freespace for the calculation by the way.
Or maybe an option to ignore the MFT reserved zone during defrag? On some drives the defrag might have gone a lot faster if Defraggler could use the large MFT reserved space when the nonreserved space is too small.