Commander Legends: Battle for Baldur's Gate was released on June 10, 2022 - this is where the Astral Dragon appeared. THEN Spelljammer: Adventures in Spaces was released on August 16th, 2022 - where the Astral Dragon saw print in a 5e product.
I think the implication is that MAYBE the same sort of "preview"/"sneak peek" is happening with the Chaos Dragon in MtG and the upcoming Planescape 5e release.
Btw, this is all AFAICT because I stopped playing/following MtG a looooong time ago, so others may know better.
I'm a bit confused about everyone here discussing astral dragons being in the Spelljammer set. There are no astral dragons in the Spelljammer set. There are solar and lunar dragons, but no astral dragons. And the astral, solar, and lunar dragons have been all separate entities since 2e, so the astral dragons didn't stealth change into one of the other types.