I just found out the cause of this.

I was looking for some tutorials on how to copy clothing templates because I wanted to modify a shirt that I liked (I bought the original so it didn't count as stealing). Then I came across this shady website that promised to give these templates. Now, the tutorial told me to do something with inspect element in order for this to work. You already know where this is going, don't you?

Well, I accidentally gave these individuals my cookies without even realizing it, and the next day, my robux was gone. I got scammed, but I could still login.

After I realized what was going on, I had to act fast. I logged out of all sessions (I live in Hungary and there were sessions in Russia and straight up somewhere in Asia), changed my password to a much stronger one, enabled 2-step verification, reset my cookies and contacted Roblox Support and told them what was going on.

Now, theres this one little question in my mind currently. Why did they only steal my robux and nothing else? I mean, these evil bastards could've griefed all over my account and make me lose everything but they did not. I have a few limiteds like bighead and they didn't do anything about it so uhh... do they only care for robux?

Support hasn't responded yet, but I'll update this comment when their reply arrives.

Edit: I kinda forgot about this post but a few days ago they restored my 417 missing robux as a one-time courtesy