I may be wrong, but if I understand the math being used to determine the alignment of the RA axis, it seems that the images being mirrored should not affect the result... or at least, if it does affect the output somehow, I'd consider that a bug. Astrometry.net is capable of handling images of either positive or negative parity (i.e. "normal" vs. "flipped,") so unless the Astrometry.net settings are incorrect, such as being told to only search for positive-parity solutions when the images actually have negative parity, they should still solve. Each of the three solved plates equates to a great circle on the celestial sphere, defined by the central coordinate of the image and its rotation, neither of which should be affected by the parity of the images, and these three great circles should intersect (or roughly intersect, accounting for error) at two points corresponding to +90 and -90 degrees declination. That's not to say that mirrored images definitively aren't to blame, simply that there's no practical reason why the system wouldn't be able to handle them, as far as I can tell.

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