Actually, the boar only triggers the extra cutscene where Astarion offers to stand watch at night, not the bite scene.
Usually, unless something pushes back his scenes (scenes with Urge for example), he'll come biting the next night after he's gone hunting.
It's unlikely you'll be able to push back his bite-recognition scene for long, unless you avoid a long rest (usually 3-4 nights). He's hungry, that's all the cards on the table right away))
So, if you see an elf sneaking into the forest in the night, he'll come to you at the next long rest.
In ye olden days (EA) this was the order the sequence followed, (it might have changed, I did notice some changes in the way scenes triggered but I didn't make note of it so it's possible):
1. There's a long rest where you see Astarion sneaking away
2. Now the boar's corpse is there
3. Inspect it, Astarion intervenes
4. Long rest and bite night ensues
It definitely didn't trigger if you didn't run by the boar back then, I remember my frustration at how low my approval was by the time of the party back then because of this.
I also remember that for my checking-dialogue speedruns I always did it in conjunction with talking to Zorru with Lae'zel in the party, as this would prompt a conversation that happened that same night right before the bite where he's doing some stargazing. I do recall they changed the way this specific scene triggers in full release, although not how. It could be in the way you mentioned.
My full release Astarion has somehow gotten bugged in about half of my runs so the only way I find out about his vampirism is when he confesses in the underdark/mountain pass.
