It's not wrong, it's just my headcanon

I'll begin with reflecting on how things could have ended with Seb’s dad: the first thing I would exclude is that he passed away, because I find it a bit hard to believe that Robin would have moved on so soon as a widow with a little kid and marry/remarry + have another baby immediately after. Mind you this is an assessment purely based on vibes, but Robin doesn’t strike me as the impulsive and restless type, she's surely feisty but also no-nonsense and hardworking, and somewhat of a lover girl (we can see her and Demetrius dancing at the pub and stargazing together). To end things with a child in the mix it could either mean that he fucked up bad enough that she didn't want to have anything to do with him anymore, or that he was the one who left (either way we're looking at a monumental fumble).

My personal headcanon is that he was an asshole left her and Sebastian, maybe they didn't have a great relationship to begin with, and for a long time already, maybe she tried to fix things but he simply didn't want to settle or be faithful, or maybe she just had to let him go and choose hers and baby Seb's wellbeing. All this to say it’s very likely that he wasn’t a great person, and while Sebastian may look like him he definitely doesn't act like him. Seb grew up angry, avoidant and with a lot of unresolved issues, and later even with somewhat of the same bad boy look, but his heart was always in the right place. If anything his anger was due to being hurt and the fear of not belonging anywhere - which is very different from not caring to belong and leaving everyone behind like his father did.

So while looking at his face might sometimes send a pang of sadness through Robin's heart, she cant help but notice how different Sebastian and his father actually are. In my Older!Sambastian timeline/au Seb gets to overcome many of his issues, so I feel like she would actually make a comparison and feel even more proud of her boy and the man he’s grown into 🥹