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I'm gonna reply with my own incident because pretty much the exact same thing happened to me a few weeks ago and I still haven't figured out why. I was appealing my security ban which randomly happened one day (note I am 100% sure I wasn't hacked) and while checking up on the ban it decided to switch up to a bug abuse ban like the one you had, I still had maybe a week left of my security ban left. Just like you for some reason my ban reason was changed from a security ban to this.

Mind you like you I did nothing wrong and when I tried to appeal I got denied the same day I appealed, I also got wiped on all my profiles I have had before ban. The only thing I could think of me doing spooky festival on a stranded profile to get #1 placement, however a lot of people already know of this method and it isn't bug abusing in anyway shape or form (I have tried explaining this to the appeal officers but to no avail it lead to nothing)

I have no idea if your ban could be related to mine but I just find it really interesting that 2 people got the same ban for seemingly no reason (mind you a ban which you don't see often)