Mashiba also had fought dirty during those fights. I bet that this is going to be the fight where he finally fights like a real boxer. This is the fight where Mashiba will no longer cheat and, finally, become a monster.
And how is he going to do it? By fighting his reflection. What he could have become, if not for his sister.
I just went and looked at a couple of the other fights and no, Mashiba did not fight dirty a single time, ever since he came back from his suspension after the Sawamura match.
In Mashiba's OPBF lightweight title match against Aaron Domingo, Domingo once he was on the backfoot headbutted Mashiba several times, scoring a down and gaining an advantage. Mashiba grew angry and almost lost it, however he remembered all the people who supported him despite and throughout his suspension and fought/won clean and legal.
And then in his fight before this current one against Juan Garcia (Garcia fights clean throughout the whole match) Mashiba was pushed to the brink and considers fouling to get an advantage, but his inner monolog reveals that now he would rather lose a fight than betray the expectations of everyone who supports him. He struggles throughout the fight, much like Ippo did in his fights, and ultimately wins clean.
These two fights, against one cheating and one clean fighter, already covered what you described and completed Mashiba's fouling character arc.
Having another test of this old character flaw of his just feels like padding, even though Rosarion is described as a bad sportman.
To top it off this fight doesn't need it. It was amazing so far and Rosario gave off Takamura vibes. Absolute asshole outside of the ring, but a great boxer inside. Would have been a great challenge for Mashiba overcoming someone like that.