I want to install i3 as I'm used to that but after installing and choosing i3 in the login window it still logs in to Mate, no matter how I try login. Is this a bug or why is it not behaving as expected?
It is a bug - https://pad.lv/1910279
You have to try solution from Replace arctica-greeter back to slick-greeter on 20.10 for better compatibilty with other desktop environments
What is your Ubuntu MATE version?
Aha, I didn't find much when I searched but it's that bug. I'm on 22.04.