The reason one who is Misachek Bikuvia is posul for eidus is because it is as if he is stealing, since his opponent had no intention of losing.
An elementary school rebbe I had said thet kfitlach (which I was told is the card game “21”) is equally problematic.
I saw boys literally crying after losing $20 (lots of money for a little kid then), which is a clear indicator they did not intend to lose.