Its kiiiind of similar to the one that happened in the 50k PC, but kind of not also.
As it was pointed out by B that he wasnt actually all in, its not accepted action that he has moved in. However, they have agreed to get it in blind on the flop. However, this then breaks another rule. Player A has prematurely exposed his holecards, therefore cannot make aggressive action. This means he automatically checks the flop. So therefore Player B can get it in blind before the flop is dealt and Player A can call the bet.
If I was called over after this, Id probably say similar to you tbh. Well rectified and nothing actually that bad in it imo. Seems like a genuine mistake by player A
This gets said a lot. Can someone please show me a reputable ruleset (i.e. not grosvenor or gala) which has this rule.