From my own experience, and that of many poly people (solo & not) whom I've asked about "outness" lately, it seems that usually this topic doesn't get discussed clearly and specifically early in a relationship.
Discrepancies tend to emerge only after the relationship is well established and there's substantial emotional investment. And what seems to be the most common way that outness discrepancies become apparent iS when a nonprimary partner inadvertently transgresses an outness-related boundary of the primary couple, which the primary couple never disclosed and often is unwilling to negotiate about.
Another common situation is when the nonprimary partner unexpectedly finds themself excluded, avoided, or demoted to "friend" status without warning or negotiation at an event or in a social setting.
There are various reasons for why people usually don't discuss outness clearly and early in poly/open relationships:
- early in any kind of relationship (even most mono ones) it's usually seen as pushy or moving too fast to make public mention of a relationship (such as calling someone your "girlfriend") within the first few months. During initial NRE it's hard to tell how serious a relationship might be in the long term. So for people with relationship experience, outness usually isn't much of an issue early on.
- People often like to imagine they're more open or braver than they actually are.
- Poly people often initially meet, fall in love & socialize within the poly community. When that's your main social context for a new relationship, it isn't always apparent that, say, a longtime poly primary couple may be considerably or entirely closeted in other social circles.
- Often people feel that that it's unromantic to clarify outness boundaries early on, that this is an NRE-killer because of clarifies limits rather than hopefully embraces possibility.
- Often primary poly couples assume that the norm is that, outside the poly community, they get the prerogative to maintain the appearance (and privilege) of mono couplehood -- so nonprimary partners "should "expect & be willing to roll with that.
...Of course, YMMV. If you've seen differently in your poly/open circles, if the poly people you know generally think very clearly about outness and discuss/negotiate it frankly and --especially -- EARLY in new relationships, I'd find that very encouraging. From what I've seen and heard about, that's usually not how it plays out in the real world, unfortunately.