As an actor, you're so in your characters own perceptions that sometimes you forget how your character is perceived from the outside. Like the actor who played 002 told me he did not realize until he watch it that scenes like his electrocution and death were not even sympathizable or redeeming the way he thought.
I think Finn really thought people would see Byler as soon as season 4 released and not be shipping Milkvan anymore.
Here he is being openly excited about Will having feelings for Mike in an interview ahead of season 4, not attempting any ambiguity
Here he is openly cringing at the ily monologue, with no regard to ambiguity or neutral opinion that you would have if you thought of it as not at all revealed.
Here he is surprised when people cheer for Mlvn
He didn't start hiding his opinion until later, he did not in initial promotion of season 4.
And all his reactions to Milkvan are like with the assumption that other people do not like them romantically. Consciously making a "yikes" face after the monologue and surprised when people cheer for Milkvan.
That one's really funny because he's saying "Mike and El's romance is cute bc season 1 was cute". He might be trying to be ambiguous, but he could also really think that about season 1. But he acts like he expected them to take something very different from that.
He just consistently in these interviews seems to act like the reaction he's anticipating from everyone around him to be Byler.
Because, like I said, we do have a tendency to mix up what we know to be true of the characters with what the audience knows.
He just filmed season 4. He was used to knowing so he forgot you didn't.
His earlier promo interviews for season 4 had an overall energy of "I don't have a preference ;D ;D *cough*byler*cough*"
Season 5 ones and conventions since are what have become more "I don't have a preference😐".
Maybe he noticed the reactions that he wasn't expecting, maybe PR talked to him, who knows.
But as an actor who, like others, surely remembers more of his character's inner world than the final product he's barely seen.....He acted as if everyone saw what he knew of Mike's experience that season.
In those first 3, openly not even trying to hide his reactions or preference. And then showing surprise when people reacted in support of Mike and El being together.