Test passes when run alone; fails when run with other methods in the same class

Debug_VFxTestFailure

@isTest
private class Debug_VFxTestFailure {
    /**
    * @description Sets up data for test methods
    */
    @TestSetup
    static void setup() {
        List<Account> accts = VFx_TestData.createAccounts(1);
        insert accts;

        VFx_TestData.insertUser('test1@VFx_VolunteerActivities_Test.test', 'Donor on Platform');

        // insert an activity
        List<Volunteer_Activity__c> activities = VFx_TestData.createVolunteerEvents(accts[0], 2);
        insert activities;
    }
    
    // this test will pass when run alone, but fail when run along with testDestroyAllowed()
    @isTest 
    static void testSaveUpdateAllowed() {
        User donorUser = 
            [SELECT Id FROM User WHERE Username = 'test1@VFx_VolunteerActivities_Test.test' LIMIT 1];

        Volunteer_Activity__c activity = [SELECT Id, Name, RecordType.DeveloperName FROM Volunteer_Activity__c LIMIT 1];
        
        Boolean exceptionCaught = false;

        try {
            System.runAs(donorUser) {
                // this is actually what I want to test, but for the sake of clarity let's just do basic DML
                //(Volunteer_Activity__c)VFx_VolunteerActivities.save(activity));
                upsert activity;
            }    
        } catch (DMLException e) {
            exceptionCaught = true;
            System.debug('e: ' + e); 
            System.debug('e.getCause(): ' + e.getCause()); 
            System.debug('e.getStackTraceString(): ' + e.getStackTraceString()); 
        }

        System.assertEquals(false, exceptionCaught, 'A Donor on Platform user can update a Volunteer Activity.');
        //System.assertEquals('Something else', activity.Name);
    }
    
    @isTest 
    static void testDestroyAllowed() {
        User sysAdmin = VFx_TestData.getUserForProfile('System Administrator');
        Volunteer_Activity__c activity = [SELECT Id FROM Volunteer_Activity__c LIMIT 1];

        Boolean exceptionCaught = false;

        try {
            System.runAs(sysAdmin) {
                // this is actually what I want to test, but for the sake of clarity let's just do basic DML
                //VFx_VolunteerActivities.destroy(activity); 
                delete activity;
            }    
        } catch (Exception e) {
            exceptionCaught = true;
        }

        System.assertEquals(false, exceptionCaught, 'A System Adminstrator can delete a Volunteer Activity.');
    }
}

No changes were made to the file between test runs.

This is the error from the test failure:

System.DmlException: Upsert failed. First exception on row 0 with id a1D8A000000Ci1XUAS; first error: INSUFFICIENT_ACCESS_ON_CROSS_REFERENCE_ENTITY, insufficient access rights on cross-reference id: []