Raphel event is triggered after the first time you finish a potential lead. among them is nettie, gut, the gith guys. It has no relation to Cazador. you can have this conversation with Astarion before even learning he is a vampire. The (kinda) simple solution is to change his dialogue if you didn't discover he is a vampire yet. of course, that would require writing and extra recording and shit. another one to the pile I guess.
I yeah I wasn't sure about the Raphael trigger, my first game I didn't take a long rest for quite a while (after rescuing Halsin) so that makes sense, but the real problem of course is that you have a conversation that doesn't make sense unless you know Astarion is a vampire and have some idea of who Cazador might be to him, indeed.
It's not a question of if you can, you can't learn Astarion is a vampire before this convo, I did test it a little. I went to one of the places you can find a boar, no boar, did a long rest that triggered Raphael, then there's a boar, so I'm pretty sure Raphael is currently what triggers being able to find a boar