Same issue here. Been on with Microsoft, they say my account is fine and the bundle I purchased is applied to my account. The bundle is “Diablo IV: Vessel of Hatred - Expansion Bundle”, which includes Diablo IV, Diablo IV: Vessel of Hatred - Expansion Pack, and the pre-order pack.
Diablo support I am finding is worthless. I have provided receipts and photos of the problem but they don’t seem to get it. In the last update they provided the following link Vessel of Hatred License Revokes. Maybe this might help someone.
Note, my issue has nothing to do with “losing access to Vessel of Hatred randomly through their play sessions on consoles.”
I have the expansion pack installed and logged into a working character and then logged back out to create a Spiritborn, which are various things I have seen people suggest. However, this did nothing, same error, or “The Spiritborn class is only available in Vessle of hatred.”
In short, Microsoft says I have the base game and the expansion. Diablo says I do not have the expansion.
Nearing a point where I may just have to move on and get a refund. plenty of content out there to play as a replacement.