I got the daily quest for this today and me and a bunch other people have been waiting for at least 30min. No spawn. In zone chat people are reporting the same issue where this boss does not spawn
It's been a old thing any time there's a Summerset event. It's certain instances, so if they're not spawning in an instance, either try instance fishing or just forgo that one.
This bug with the Summerset griffon bosses when they do not spawn in certain instances has been happening for years unfortunately. The workaround is to leave Summerset and return in hopes of getting a different instance that has a working spawn time for the griffons but there is a chance of getting the same bugged one you just left.