When I run the test my debug lines show that the values for those fields are the same for both oldOpp (containing the Trigger.oldMap values from my Trigger) and newOpp - o. - (containing the Trigger.newMap values).

When I run the test my debug lines in the isUpdate section show that the values for the Ramp_Profile__c & CloseDate fields are unchanged.

@isTest
public class WE_OppUpdatesTest{
    
    @testSetup
    static void dataSetup() {
        
            List<Opportunity> opportunities1 = WE_TestDataUtility.createOpportunities(
                25,                                 // noOpps
                '<*>',                              // identifier
                date.today().addMonths(1),          // closeDate
                'Standard');                        // rampProfile

            insert Opportunities1;
    }
    
    static testMethod void testOppUpdates(){
        
        List <Opportunity> opportunities = [SELECT Id,RecordTypeId,Name,CloseDate,Ramp_Profile__c FROM Opportunity WHERE Name LIKE '%<*>%'];
        
        Test.startTest();
 
        List<Opportunity> OppClsMnths = new List<Opportunity>();
        
        for(Opportunity o : opportunities){
            o.CloseDate = o.CloseDate.addmonths(1);
            o.Ramp_Profile__c = 'Immediate';
       
        }
        update opportunities;
        Test.stopTest();
    }
}

@isTest
public class WE_OppUpdatesTest{
    
    @testSetup
    static void dataSetup() {
        
            List<Opportunity> opportunities1 = WE_TestDataUtility.createOpportunities(
                25,                                 // noOpps
                '<*>',                              // identifier
                date.today().addMonths(1),          // closeDate
                'Standard');                        // rampProfile

            insert Opportunities1;
    }
    
    static testMethod void testOppUpdates(){
        
        List <Opportunity> opportunities = [SELECT Id,RecordTypeId,Name,CloseDate,Ramp_Profile__c FROM Opportunity WHERE Name LIKE '%<*>%'];
        
        Test.startTest();
        
        for(Opportunity o : opportunities){
            o.CloseDate = o.CloseDate.addmonths(1);
            o.Ramp_Profile__c = 'Immediate';
       
        }
        update opportunities;
        Test.stopTest();
    }
}