I think a few different races played into that. There was definitely a Tour stage (Rodez 2015?) where GvA followed Sagan’s wheel to the win, and iirc Greg was in a position to close the gap to Sagan at the Richmond Worlds, but didn’t pull. So it wasn’t always directly against Sagan, but they rode a lot of the same races and Greg didn’t always throw caution to the wind. I don’t think you could point to a single standout race where Greg sat on and they both lost, but there were races where everyone sat on Peter’s wheel and lost, and GvA, as his main rival, caught the brunt of it.Right, makes sense. I don't know from where that reputation has come, either. Perhaps because he won all the classics in 2017, where Sagan was the most vocal about lack of cooperation. But Greg wasn't the one who was at fault, at all.
By contrast, Kwiatkowski actually did beat Sagan a couple of times, including at MSR, but he generally pulled (except at MSR) so got a break from the Sagan-fans.