So, I just made a new seasonal character (I own base game and VoH). I will list everything I did so, somebody else might be able to replicate this magic trick.
Steps
- Male Barb, Seasonal, Selected Start with Vessel of Hatred Campaign Complete
- Spawned in and ran to Gelena
- Skipped through the conversation dialogue
- I was tasked to go explore the area to the west in Gea Kul (Starting the Oaths and Debts Questline - Help Gelena find the source of the whisper). She will follow you. Mount your horse and she will disappear when she mounts her horse.
- You are now a magician as well.
I’ve left the game and came back and, exited and restarted diablo but the bug remains. Gelena still disappears every time I mount the horse. Her ghost horse still kicks up dust and, she still speaks to you. Enjoy!