actually its not like that. you cant change stances while you are getting punched and have to switch in between the hits. some other things might cause a position change too, for example when your attack gets dodged or someone walk around you.
I think its because you can't block with your back to the opponent. I guess. I honestly dont know, but I DEFINITELY agree; its super annoying. It's hard to initiate a specific counter-attack when my stance keeps changing.
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To answer your question, we would need to see video accurately depicting what you're describing.