Question about Chaotic Swell

I was using an eternatus deck and had a Chaotic Swell on the board, and my opponent played path to the peak and it forced mw to discard the extra mons from my bench before it was discarded. Is this how it's supposed to work, or is it a bug? It doesn't say the new stadium doesn't work for that second it's on the board but the text gives me the impression that it shouldn't even have the chance to take effect first.