Unconventional approach to getting a formula field to update in apex. The tradeoff is it consumes extra limits for the queries and DML operations for update and rollback etc. Example is non-bulkified but could easily be made to be so.

apex.cls

    // get existing record
    SBQQ__Quote__c existingQuote = [
        SELECT 	formula_Field__c 
        FROM 	SBQQ__Quote__c 
        WHERE 	Id = 'xxxxxxxxxxxxxxxxxx'
    ];
    
    // capture the current value of the formula field
    Decimal oldFormulaValue = existingQuote.formula_Field__c,
            newFormulaValue,
            reQueriedOldFormulaValue;
    
    // generate rollback point
    Savepoint sp = Database.setSavepoint();
    
        // update fields that impact the formula
        existingQuote.Formula_input_1__c = 5000;
        existingQuote.Formula_input_2__c = 10000;
        update existingQuote;
    
        // query for the new formula value
        newFormulaValue = [
            SELECT 	formula_Field__c 
            FROM 	SBQQ__Quote__c 
            WHERE 	Id = :existingQuote.Id
        ].formula_Field__c;
    
    //now that we have the updated formula value we can revert the changes in the DB
    Database.rollback(sp);
    
    // requery the formula field to help prove the rollback happened
    reQueriedOldFormulaValue = [
        SELECT 	formula_Field__c 
        FROM 	SBQQ__Quote__c 
        WHERE 	Id = :existingQuote.Id
    ].formula_Field__c;
    
    // assert that the rollback happened
    System.assertEquals( oldFormulaValue, reQueriedOldFormulaValue );

    // assert that we have the updated formula value
    System.assertNotEquals( oldFormulaValue, newFormulaValue );