Alright, I have been asking around on this to try and figure out how it works. One of the people is Rogue Warrior speedrunner Traz64. Since both games are made in the same engine it could might mean some of the same mistakes are in both games, so we could help each other out here.

What he had to say after watching the stunlock was the following: "Traz64: It looks like your hits are moving him slightly. Perhaps this is interrupting a scripted action he takes when he spawns and because he drops in he doesn't attempt to reroute himself back to that point. Enemies in rogue warrior have a scripted sequence to go through before their AI kick in (generally running to a specific cover point) and we normally use this to run past them before they start shooting. It could also be frame perfect to cause that displacement however."

I hope with this info we can figure this stunlock out and optimize this category!

The best luck I've had on it is lining up directly behind him locking on and mashing light attack. That's what works for me at least. If I'm to his sides he falls off, but directly behind is fine. I'm not sure if this helps in any way, but thought I could pitch in.