Far as I'm aware it hasn't happened before (which I admit isn't a reason for it not to happen) but also that the PTR is simply testing how the rest of the game will work sans the expansion.
I don't have much issue with it myself but its never been something I expected. It'd be like expecting to be able to test the rest of expansion content on the PTR. You're not expecting to be able to access the Dragon isles or dragonriding on the PTR, are you?
Granted it is a bit odd that you cannot test a Dracthyr on the PTR, but this is also how the game is supposed to function if we assume you will need to own Dragonflight to be able to make a Dracthyr in the Pre-patch. PTR has no way to actually simulate owning the expansion, and given this is a once per expansion PTR thing (or even once per every other pre-patch PTR realistically) I don't see why Blizzard should have to bother. They already get plenty of testing done on it in the Beta.
The world revamp dream will never die!
I swear, if wowhead would post instead: "surprisingly there are no dragon isles on PTR in this build, players are angry" we would have posts how Blizzard is awful for not doing that xD.
That didn't bother me much cause at that point in the story you're evil and mind-controlled by the Lich King - it bothered me way more when the Kirin Tor has you torture that Malygos-aligned prisoner up in the Borean Tundra. First time I played through that was as a human paladin in the WotLK beta, it was very surreal when the quest giver said that the Kirin Tor has rules against torture, but you don't, wink wink.