Hello, I just finished the game moments ago and I think the posts here have pretty much laid out all my thoughts much better than I could. Just going to add what I think about this.
Prior to Novigrad I was looking forward to meet Triss again, wondering what kind of reunion scene could they have in store, only to find my Geralt had broke up with Triss off screen without the player's consent. Honestly I think it turned out not so bad, since I figure Geralt might need a break to examine his feelings now that he's going to actually meet Yen again. I think it also introduced new dynamic between Geralt-Triss, an awkward (and admittedly rather cute) phase that we've never seen before with Triss and also not in Yennefer's path. I mean, between the first scene with Kingbeggar and the scene at the warehouse, I can very much feel the tension pouring out of the screen. Awesome job on that one CDPR.
But then like everyone here already said, after the too early romance lock-in quest there's no more growth in their relationship. No proper reaction from other characters and Geralt still has to behave as if he's still with Yen. I quite like Yen as a character and I don't mind if she's the one Geralt goes with in the main missions directly related to Ciri because she's supposed to be Ciri's mother figure so it's rather odd if Triss replaced her there, but can we at least get one or two more side quests to make the flow of Geralt-Triss relationship more natural? Or something else (reaction from other characters or an item maybe) to feel that Geralt is actually in a relationship with Triss? Triss was also gone for a big portion of TW2 but there's always this feeling that they're connected, at least for me. I know in TW3 Triss is very much only a support character while the Geralt-Yen-Ciri triangle is the main star, but surely a better balance can be reached?
I really hope CDPR will give Triss' fans something to fix the imbalance, whether in DLC or EE. Even little things like journal entries that actually say what happened can go a long way. Items (Rose of Remembrance, why you wasted that item like that?), extra dialogue with Triss and comments from other characters, or if you guys are generous a new side quest and cutscene or two. I know a DLC dedicated to a single character is not going to happen, unless there's a plan for something like Citadel DLC.
P.S. I initially planned 2 playthroughs for TW3, one for Triss and one for Yen, but after seeing what happened I don't think I can do the Yen playthrough in the near future. Sorry CDPR.
P.P.S Wall of text!