It's possible you need to appeal the Medicare decision - a brief search shows that Part A should cover some in-hospital dental work:

Since I still can't post links, I suggest Googling "medicare paying for dental"

medicareinteractive (dot) org has more details than the official Medicare site and specifically calls out dental work related to oral cancer as being covered.

I suspect Medicare is like most insurance - the person reviewing the approval just hears "Dental" and says "Not covered" - even though it's not true in some cases.

My husband reminded me that he held off paying the hospital’s dental bill while disputing it and the hospital eventually reduced the bill by 50%(this was not Mayo).