I have something similar with one of my bunnies
It's a dental issue
We got our bunny from an animal shelter
He was eating fine then started to go off his food, very unusual for this rabbit
He could still eat some pellets (hard oxbow, as it was the only thing he would eventually eat) but refrained more and more from other foods
Our rabbit savvy vet looked in his mouth and discovered he was missing a tooth towards the back of his mouth (very hard to see without instruments) which meant the opposing tooth was growing longer and eventually was so long it impacted his ability to handle foods that those teeth normally dealt with
They snipped the elongated tooth and as soon as we got him home and put food out he was back into eating it (a little cautious at first due to habit of not touching it but within 10 mins the old habit was all forgotten)
We were told we would need to do this very 6 - 10 weeks. We forgot about it and 6+ months later he started refraining from food again, thankfully one of the kids remembered the previous tooth issue, so off to the vet again, snip went the elongated tooth and he was back gutsing his food again (this bunny loves his food)
Hope this is the same issue with your bun as it's an easy fit, assuming your vet can both see what's going on in their mouth and is able to fix it