I have a formula field in OpportunityLineItem -'Is Available'(IsAvailable__c) which takes values from product object's checkbox field -'Is Available'

FORMULA : PricebookEntry.Product2.IsAvailable__c

When I tried to create dummy OpportunityLineItem record  within test class 'Is Available' formula field in OpportunityLineItem is returning me value as false,where as I am setting this value True at product level. Because of this major part of my class is not getting code coverage.
Can anyone please help me  to find what I am doing wrong here.
 

static testmethod void TestMethod() {

        
        //Created A dummy Account and Opportunity records I.e  Acc & Opt
        
		//getting standard pricebook id
        Id pricebookId = Test.getStandardPricebookId();

		//Created a product
        Product2 prod = new Product2(
             Name = 'Product X',
             ProductCode = 'Pro-X',
             isActive = true,
             IsAvailable__c = True // setting field value as true
        );
        insert prod;
        

        //Created pricebook entry
        PricebookEntry pbEntry = new PricebookEntry(
             Pricebook2Id = pricebookId,
             Product2Id = prod.Id,
             UnitPrice = 100.00,
             IsActive = true
        );
        insert pbEntry;
        
        //created opportunity line item.
        OpportunityLineItem oli = new OpportunityLineItem(
             OpportunityId = opt.Id,
             units__c = 25,
             Type__c = 'No. Users',
             PricebookEntryId = pbEntry.Id,
             TotalPrice = 200,
             Quantity = 1
        );
        
        insert oli;
		
		system.debug('oli.IsAvailable__c ='+ oli.IsAvailable__c ); // This is returing me value as false

		Test.startTest(); 
		
		//calling my required methods here
		
		Test.stopTest();
	}