DAO romance works differently than DA2's does. You're locked into Alistair's romance after the tent scene, I believe. (It's been a while and I don't remember exactly.) In DA2, romances weren't locked in until the scene with the companions before the final battle. Hence the reason that Seb's romance wouldn't be complete if you elected to let Anders live.
That's not exactly correct. Technically, all DA2 romances "lock in" (in the sense that you are locked out of all other romances) at different points - Anders' and Merrill's are locked in by letting them move into the manor after the sex scene in Act 2, Fenris' and Isabela's are locked in when you agree to get back together during their Questioning Beliefs quests in Act 3, and Sebastian's is locked in when you either marry him (friendmance) or "get engaged" of sorts (rivalmance) in Act 3. By the time you get to the final goodbyes in the Gallows, you're either married or engaged to Sebastian, or he's walked out on you entirely - his romance is complete, there's no further to go.
I questioned the breaking of Sebastian's romance tile if you spare Anders a lot during the Keep's beta because it used to be a popular headcanon among people who didn't want to kill Anders that their Hawkes would later talk to Sebastian, calm him down and make up, everything would eventually work out, etc. And what the devs said was that this was a writing room decision supposed to signal that Sebastian leaves Hawke if she spares Anders, no matter what (if they're married he annuls it, if they're engaged he breaks it off, etc). That's kind of what sucked about it, people invested a lot in their "everything will be alright" headcanons, and the writers said nope it won't, it's literally do or die.
But in technical terms, it's not that Seb's romance doesn't complete - the romance itself is completed in-game; it's just that you get a failure state, and rather than make yet another tile just to signal that you romanced Sebastian once but then he left you, they decided to just let Anders' tile revert your romance one. I mean, in theory I get it, it's a ton and a half of background work to add more and more tiles that react to each other. Adding more tiles to Alistair makes sense because he and his romance are so popular, tons of people will want to detail every last breath of their romance - adding more stuff specifically for Sebastian when his romance is probably the least popular one in the entire series and will have the least amount of people actually using his tiles is harder to justify. I don't like it, but I get it. They weighted whether it was worth the time/effort/resources and found it wasn't, one tile reverting each other was enough to send the message.
It sucks, but at the same time... I still thank the Maker every day that they even acknowledged Sebastian's romance was real, to begin with. You can't detail your Sebmance as extensively as Al's, but just the fact that Seb is there at all with a heart tile of his own is already a victory.