When setting a instance variables value from within a batch context the value is not retained when testing.

Example code:

public class FinishLogic {
    public Integer val { get; set; }
}

global class UserBatch implements Database.Batchable<sObject>, Database.Stateful {
    
    private final FinishLogic fl;
    
    public UserBatch(FinishLogic fl) {
        this.fl = fl;
    }
    
    global Iterable<sObject> start(Database.BatchableContext bc) {
        return [SELECT Id FROM User LIMIT 1];
    }
    
    global void execute(Database.BatchableContext bc, List<User> scope) {
        //do some stuff
    }
    
    global void finish(Database.BatchableContext bc) {
        fl.val = 5;
    }
}

@isTest
public class TestUserBatch {
    
    static testMethod void shouldReturnFive() {
        final FinishLogic fl = new FinishLogic();
        final UserBatch ub = new UserBatch(fl);
        
        Test.StartTest();
        Database.executeBatch(ub);
        Test.StopTest();
        
        System.assertEquals(5, fl.val);
    }
}

The test should pass as the value should be 5 but instead returns null. This is a simple example for demonstration purposes, but this behavor is important to test the correct functionality of a batch. 

If I add the line "System.assert(false, 'executing finish);" to the batches finish method the test does correctly fail with the error, therefore the method is being called. Does anybody know why this wouldn't work, or is this a known bug?

Thanks