you have proof that its not a set seed

Mod note: As you can see in the video, just changing the difficulty and mode in the lobby doesn't make the seed completely random. (You can see that only the penultimate character has changed) So this time I am fixing it to Set Seed. It looks like the player you're co-oping with is overriding the seed. Read the rules on how to randomize the seed. Also, check the measurement timing of the timer in the same way.