Honestly I started in Shadowkeep so I was not aware of previous lore that they weren't a couple, though Saint-14 going into the Infinite Forest to find Osiris and then Osiris bending time to try and save Saint-14 does make equal sense for love and/or camaraderie. But someone mentioned the Devil's Ruin "mission" where we have both Saint-14 and Osiris in call together for the first time and watching it now, their tone is strikingly different than it is currently. It is much closer to how close friends would joke with each other; Saint-14 telling to Osiris to leave the call, -blam!- "make me" and his reply is "you would not survive that, (but) you make me laugh so you can stay." Not quite the tone for lovers, at least how I understand it. Nothing wrong with them being lovers however, again it does make some semblance of sense but it does indeed appear to be a retcon to fit a narrative and that in itself feels wrong. Still, not a fan of love stories to begin with and it seems like we are going to end up with a wedding at the end of this Episode, just based on how the writing is going.