The problem is that DPC has many such moments. There are canonical moments that are logically explained and there is a choice of players that looks strained. This is an illusion of choice. I wouldn't be surprised that even if we didn't make a sex video with Jade, it will turn out that Burke has this video, but with another student, for example. This is logical. DPC is easier to develop this way. He wouldn't do 10 separate branches when he could do two or three.
That's fine though.
All I'm saying is that Sage would have no reason to fight with the MC. I'm sure it'll come up at some point if you decided to go the Jade route while on the Sage route, but if you didn't choose to go the Jade route, then the MC and Sage would have nothing to fight over.
It is difficult to know if the MC and Sage are fighting, since I assume the same thing happens at the end on all routes. I tested it out and it happened while on my Jill route as well.
Sage may just be mad, because the MC is taking his sweet time in the bathroom while there is important stuff being discussed outside and she needs support, as the MC promised he would be there for support.