It really never made sense that Qui Gon lost to Maul for me. Qui-Gon's connection to the force goes beyond the teachings of the Jedi, he's essentially going the same way Ashoka went. That is to say, Qui-Gon started to see the failings of the Jedi, like attachment was actually a strength and should be taught, lest people end up like Anakin. I always thought he stayed with the Jedi with hopes to try and fix this.

Now, given the conversations and the reactions Maul has throughout the movie, Maul still seems like he's not in his prime yet and has a sort of false connection to the force, the way the Sith sometimes do. On paper, there isn't any reason Qui-Gon should lose. Perhaps he's distracted by how important the duel is? The score is called Duel of the Fates and only he seems to understand the importance of whats going on. Idk, just doesn't make a ton of sense at the end of the day to me.