EDIT: I found out that I need to read patch notes next time before jumping to conclusions.
I’m not sure if this has been posted before, (if someone links me to another post I will delete this one). I have a little bit of a story before I state the bug. Some time ago, a little after T4 was released. I wanted to.master Slim. I put so much effort into mastering him, (but anyone that has tried knows what I am talking about). I was stuck on his level 3 Spore Cloud. I tried 3 days straight with little progress to the Spore Cloud becoming mastered. I got annoyed, so I stopped trying to master him. A couple days ago, I was changing my emblem when i noticed I had slims final emblem unlocked, (the one you get for mastering slim). I quickly checked to see if I had Slim, and sure enough he was mastered.
TL;DR Slim released, I try harder to master him. He was hard to master so I quit trying. One day I somehow mastered him without even playing him.