The question is similar to Autodesk Refresh Token keeps Expiring

Kindly check your code logic. The refresh token is valid for 14 days and can only be used once. If your code calls it at any time and fails to register a new refresh token then the used refresh token becomes invalid.
Also check if you are changing scopes i.e. the scopes used to get the original token are different from those used to get the refresh token.