While this is certainly a valid form of romance, it’s more accurately described as “non-exclusive simultaneous relationships” than a single “polyamorous relationship”.
Some people really do live in multi-partner committed households, but those seem most often to be dominated by a single person, such as fringe Mormon polygamy. And the most common form of "polyamory’ is probably “affair-tolerant monogamy.”
It’s a big complicated world, and variations of how humans with form intimate relationships fills all possibilities when there is no enforced legal prohibition. (And,.sometimes, even then.)
No apologies necessary*. I certainly wasn’t trying to offend, just be accurate in model setting.
A more accurate umbrella term for “affair tolerant monogamy” would probably be “non-monogamous”, with the dividing line between that and “polyamory” being exactly what you said : all persons in the relationship cluster knowing the stances of all other participants.
Accurate and non-offensive terminology can be hard.
It does circle us back to OP, though. The answer to “what happens when one couple breaks up in a polucule” is a loud and emphatic that depends on what type of polucule you’re in.
(*: no apologies needed from you. To the extent that I caused you any distress I sincerely apologize. Causing pain was not at all my intent.)