trigger TestTrigger on TestObj2__c (after insert) {
    Set<Id> ids = new Set<Id>();
    
    for(TestObj2__c a: Trigger.New) {
        ids.add(a.Account__c);
    }
    Account[] acs = [select Id from Account where Id in :ids];
    for(Account b :acs) {
        b.TestField__c = 'BECOMING A PROGRAMMER';
    }

}

Expectation: After I create a record of object TestObj2__c, a field of it's master object Account will get populated with "BECOMING A PROGRAMMER".

It does not do that though. Even wrote a test method for this, says the same:
 

@isTest
public class TestTestTrigger {
    @isTest static void td(){
        Account a = new Account(Name = 'fun');
        insert a;
        TestObj2__c b = new TestObj2__c();
        b.Name = 'Fun';
        b.Account__c = a.Id;
        insert b;
        
        Account isPopulated = [select TestField__c from Account where id = :b.Account__c];
        System.assertEquals('BECOMING A PROGRAMMER', isPopulated.TestField__c);
    
    }

}

Error Message	System.AssertException: Assertion Failed: Expected: BECOMING A PROGRAMMER, Actual: test
Stack Trace	Class.TestTestTrigger.td: line 12, column 1

Can anyone tell me what's missing in the puzzle?