David, like Iridium I have not heard of this before. I know it is possible to set the converter to rotate the image 90 degrees or to flip the image (mirror) by editing the BMPADJUST setting in the amvtransform.ini
within the AMV Converter folder as follows

BMPADJUST=0 - Normal
BMPADJUST=1 - Flip vertical
BMPADJUST=2 - Flip horizontal (mirror)
BMPADJUST=3 - Rotate 180 degrees
BMPADJUST=4 - Rotate 90 degrees clockwise
BMPADJUST=5 - Rotate 90 degrees counterclockwise

but flip and rotate is new to me. Check to see yours is not set to 3 but as you get different results on different players I don't think this will help.