CreateDefaultAssociatedGroups issues in sandboxed feature

Hi,
I'm trying to call CreateDefaultAssociatedGroups method in sandboxed feature.

I first call the BreakRoleInheritance method and then the CreateDefaultAssociatedGroups, and nothing happens but the BreakRoleInheritance.

if (!web.HasUniqueRoleAssignments)
   web.BreakRoleInheritance(false);

web.CreateDefaultAssociatedGroups(web.Site.Owner.LoginName, null, web.Title);
web.Update();

So I noticed that if the 3 spweb.Associated*Group variables are not null, the method CreateDefaultAssociatedGroups does nor create the web groups.

At this point I tried to set to null the 3 variables before the call to CreateDefaultAssociatedGroups method, but I got a null reference exception.

web.AssociatedVisitorGroup = null;
web.AssociatedMemberGroup = null;
web.AssociatedOwnerGroup = null;

Microsoft.SharePoint.UserCode.SPUserCodeSolutionProxiedException: Object reference not set to an instance of an object.   Server stack trace:
 at Microsoft.SharePoint.SPWeb.AssociatedVisitorGroup__set(SPGroup value)
 at Microsoft.SharePoint.SPWeb.set_AssociatedVisitorGroup(SPGroup value)    

At this point, I figured out that the only way to create the default groups is to initially create the web with inherintance broken, and this time all worked well!

The questions are:
- There is some bug?
- Why the BreakRoleInheritance method does not clean the Associated*Group variables?
- Why CreateDefaultAssociatedGroups does not create the groups if those variables aren't null?
- Why I can't set null the variables?

This beahavior is both in Sharepoint 2013, so the last question: why I can't use this API in Online environment?