I had the same issue, lucky for me I learned from the previous Dragon Age games to save often.
If you walk near the entrance and trigger the Mage cut scene, Belle will disappear (as well as the guy next to her in his stall). So, the only way to get her, is by speaking with her right after the templar cutscene ends, and before you explore upstairs, fast travel, complete companion quests, etc....basically she glitches out of existence if you do anything else.....
Hope this helps 🙂
The Prima Guide listing of many more merchants and them not being there...that I have not figured out yet....and I really need more velveteen :/
