Just because one is polyamorous and not hiding it, does not mean that they are practicing healthy polyamory. Unfortunately many individuals use polyamory as a framework to attempt avoid consequences for shitty behavior like cheating (yes, you can cheat while polyamorous!) or as a way to cope with their inability to actually commit to anyone or simply to get their narcissistic needs met. In many cases they get a certain set of needs or most of their needs met by a primary partner and treat secondary partners more as toys or one-dimensional with regards to the missing needs that their primary aren't giving them. I've met and dated people who ultimately were not very interested in a truly collaborative relationship (hard boundaries in inappropriate places causing power dynamic issues) while making no effort to contribute equally to a relationship. Part of that is propped up by the reality that managing a breakup is easier when you have other partners who can and will provide emotional support, or as stated earlier primary partners who already provide most of your needs.