Did my copy of the game get Downgraded?

I got a “free copy of Diablo 3” when I purchased an Annual Game Pass From Blizz back during Cataclysm [WoW] - It came with Tyreal’s Charger.

I’ve got the Charger in WoW but I can only play Act I of Diablo. Was the “free copy” just act one or do I need to reach out to support?

I have no idea what that is. You should probably contact support unless there is some kind of fine print that came with it.

Sounds like you are still playing Starter Edition. Did you redeem the code for DIII?
If you did, make sure you are signing in with the correct email address.

Did you get this way back in 2011? If so, you may need to contact customer support for assistance. Even the FAQS related to this are no longer on the Blizz site.