Navigating Polyamory's Challenges in Intimacy and Boundaries

I’m a doctor who’s been in an open relationship for two years with my primary partner, Alex. We both agreed to polyamory because we found that monogamy just wasn’t enough for us individually or as a couple. The dynamics have worked well so far, Alex is also seeing someone else named Morgan and I’ve got my own connection outside of our shared lives. The challenge arose recently when I discovered Alex had been engaging in activities with Morgan behind my back that weren't explicitly discussed beforehand, and this includes sexual role-play involving taboo themes. Initially, it wasn’t clear to me whether these activities were consensual and framed as fictional narratives or if they crossed real-life boundaries between them. This led to a heated discussion where both Alex and I felt hurt and misunderstood. The act itself that caused the issue was when Alex described how they had a scene with Morgan involving explicit verbal commands and physical dominance play, which is something that we’ve never explored in our relationship due to my personal comfort limits and clear prior agreements. Alex assured me it wasn’t about real domination or control but about role-playing and emotional release. However, the lack of transparency before engaging in these scenes has left me feeling wary and questioning my trust. I know this situation is complex as there’s no black-and-white answer for polyamorous relationships, where consent and understanding are paramount. My concern isn’t about jealousy or possessiveness; it's more about the integrity of our agreements and respecting each other's boundaries without assumptions being made. I’m also mindful not to stifle Alex’s exploration within a safe and consensual framework with Morgan. How do others handle these scenarios? Is it normal for there to be misunderstandings around role-play dynamics in non-monogamous relationships, or is this something that requires open dialogue and clear communication at all stages of the relationship development?