Sort of, maybe, probably not.

The study you're citing doesn't actually have a methodology that one can interrogate about how it reached this figure, and there's some fairly common patterns like opening a marriage in response to infidelity, that would create a misleading correlation between open marriages and unstable ones.

That's vastly different from entering into a relationship with the intent to be non-monogamous (and perhaps already having multiple partners), but is probably much more common, because, for whatever reason there's less of a taboo on sleeping around while married than on having multiple committed romantic relationships.

There is also, you know, just paying attention and drawing our own judgements from what we see with our own eyes.

Not that normal marriages are sunshine and rainbows. But I have yet to see a poly marriage with kids that wasn’t deeply disturbing. Though of course the primary folks said everything was fine.

That’s entirely possible. It’s also entirely possible that something about starting a relationship monogamously and then transitioning has some kind of protective effect vs starting poly.

We don’t have enough data to tease out the cause. We can only say that open marriages, for whatever reason, are less stable than closed ones.