Our rabbit was eating very little hay when we got him. We tried cutting back on pellets and – he lost weight. He’d chew willow stick balls to bits, though. After we had him about a month, we found an abscess and took him to the vet to discover he also had a serious tooth problem – an upper tooth with no matching bottom tooth which had grown out of control. The vet ground it down and since then he has eaten more hay (still not enough but more) and lost interest in the willow chew toys.
So yes, a bun with dental issues can definitely avoid hay and still chew hard things. According to my vet, it’s related to how they chew. Hard foods are chewed with a side to side motion while hay is ground with the jaw moving in all directions. There’s no guarantee it’s a dental problem but it’s well worth looking at. Keep in mind that if he’s eaten little hay for a long time that could have caused dental issues to develop or worsen.