Documents are not updated to the next major version in a Vault Bulk Document State Change.
Check the entry actions that are set on the bulk action.
A bulk action follows the entry actions for that state. The entry action: Set new major version needs to be configured for this to work in a bulk action. It is needed for an individual document state change.
Navigate to: Admin --> Configuration --> Document Lifecycles --> [Lifecycle] --> States --> [State] --> Entry Actions.
Vault Help Documentation: Defining Document State Entry Actions