I've been running into sites using similar methods to implement what appears to be a provably fair way to determine outcomes which could be manipulated by casino owners without detection. What makes them different from provably fair games that are actually provably fair is the use of a 'server secret' which is a predefined state of the game (order of cards, a number to be rolled, slot reel positions, etc.) which is then manipulated according to the hashed client + server seeds. The problem is, the 'server secret' is generated without any transparency - completely behind the scenes.
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It's not fully provably fair but it's quite provably fair since they can't change the outcome depending on your final choice and you can check which outcome was the winning one at the end. Then if they only create poor outcomes for you each time, you will quickly see it and notice that the true RTP is way lower than the expected one. They could also try to analyze how you are playing from the previous rounds but it's up to you to change the way you are playing in order to defeat this kind of ploy.