So, I have a Word document. Pages from 1 to 6 are A4 paper size format, then I have a section break, so the rest of the pages 7 to 12 are A3.

Orientation of all sheets is portrait.

But, when I am printing duplex, printing on both sides (left bind), it prints pages 1-6 appropriately, but sides on the next paper, 7 and 8, are upside down. If I turn on top bind, duplex pages 1-6 would be upside down, and pages 7-12 would be ok. But I can not make it so that 1-6 sheets would be printed left bind and 7 - 12 would be top bind. But I dont think this is really a problem. A problem is that there seems to be some bug in printer or word, when printing diffferent page sizes after section break. Anyone has any idea how this problem could be solved? I am using printer Konica Minolta bizhub 283. office 2010.

This may have more to do with your printer driver. Check if your printer allows you to select a finishing option like "Print with Mixed Paper Sizes/Orientations" which is usually available in your printer properties.