Has she had bupredine before without this reaction? If it's the same as buprenorphine here it may be making her feel too sluggish to eat, or she could have a sensitivity to it. I have to take Ebon for a dental tomorrow and will have to ask the vet what kind of pain cover she'll have because one of them makes her a zombie and she presents as more uncomfortable when she's given it, rather than less.
How does Sophie do with syringe feeding? This might be something you should consider if she continues to be inappetant. Getting nourishment in after a procedure is so important. I understand your distress that the dental doesn't seem to have resolved the issue.
Years ago my Fiver drew a vet who clipped his teeth instead of burring them and she gave no pain medication after his op. He took some time getting back to eating afterward and I had to go back and ask why and demand metacam for him. I never used her for his procedures again because she didn't think rabbits needed analgesia.
Doubling up vibes for Sophie and you. I really hope things improve for her. xxxxx
ETA: Laser therapy on her jaw might be helpful too if your vet recommends it. She'd need some kind of eye protection during the treatment.