Class is set in the item record but is not automatically populated in the Credit Memo

Class can be set in the Item record and this defaults in the Class field in transactions. 

In a Credit Memo, if the Class field is not automatically populated with the value in the item record check if the Credit Memo is created from a Return Authorization.

When linked to another transaction, the class in the source Return Authorization determines the class in the Credit Memo. If no class is set in the Return Authorization, the class field will also be blank in the Credit Memo.