Something similar, though much less extreme happened to me. My Australian girlfriend got a year long visa to stay with me in the US. The immigration officer detained her for 3 hours, and eventually told her she could only stay for six months. (This was a long time ago, late 99s, so I can’t remember the justification or how he had the authority to alter a visa she had already been granted. He may have been lying.)

He himself wasn’t born in the US, a detail which really pissed me off at the time.

How can someone whose job is to check your visa have any power to alter it?