With due respect, you have jumped a little too early to a

WRONG conclusion.  
 

That entry stamp on the passport is effectively the visa, so it is definitely NOT what you called a transit without visa.

 

in addition, our last airport was HK before coming to China mainland and it was Osaka after leaving China., so it is definitely NOT a round trip as you called it.  When thinking outside the box, one might end up having a very different conclusion.