Yeah, this seems to be a bug. When you peek in the room, he's alone on the bed watching porn on his phone, but a little later they're talking as if the mc saw them having sex on the bed. It's a problem with the Jasmine flag because when the flag is false, she talks about how Cornell was just trying to seduce her and she liked the mc getting jealous and wanting him to get back to the room faster.
I doubt it's intended, but the sex scene with Jasmine (that only appears if the Jasmine flag is on) doesn't add a cheating point that time. Might just be intended because Julia isn't around to catch them, but probably an oversight by the dev.