I logged into the launcher to check status on the Microsoft migration and noticed Minecraft Dungeons has it's own launcher. I downloaded it and logged in first with my main Minecraft.net account which showed I don't own the game, (which I do), then I logged out and tried my Microsoft account and it also wanted me to buy the game again.
I've already purchased the full heroes edition and all DLC. When playing Minecraft Dungeons through the Xbox App, it shows I own the full game. (Using my Microsoft account). but not in the Minecraft Dungeons launcher.