The crazy thing is my 500USD deposit won, and won from there on - so I never actually touched the bonus. Yet they decided to cancel all of my winnings.
This is one of the screenshots you posted in OP:
https://gyazo.com/36c2425f093657035a4be44bf22a1905.
It shows, what I assume, is your playthrough (wagering) of the welcome bonus. At the time you took the screenshot, you had wagered the bonus 0.04% out of 100% of whatever wagering requirement the casino has. According to their
website, it's 10x on sports and 35x on casino games.