Optimistic locking in CM does not work

Steps to reproduce (in XPages project):

1. Login as admin
2. In a separate browser window, login as author
3. Go to the experience page in the Experience manager (in both windows)
4. Change the layout as author (drag and drop one component somewhere else)
5. Change the layout as admin

Expected result:
Admin gets a warning that the layout cannot be changed, since it had been changed by another user.

Actual result:

The last change done is the change which is accepted. So, the other user does not know that her changes were modified.