On 9.5 (I never tried previous versions), after writing to a mirror print, it automatically clears when I open a crafting bench, or pick the print up (such as to move into the personal 2x2 crafting area). Because of this it can't be crafted with the positive/negative prints, and so mirroring doesn't appear to be possible right now.

Fixed in 1af36e9

New build should include this, 9.6

That was fast! Keep up the awesome work. LOVE this mod.