Adding new items in a collection bloc removes the previous ones.
Steps to reproduce:
Install a clean version of eZ Platform 3
Create for instance 4 articles.
Edit the home page and add a collection bloc.
Select 3 articles and add them to the collection bloc, and save and publish.
Edit an other time the home page, edit the collection bloc which already has 3 articless, browse and add an other article.
=> The 3 previous articles have been removed
This behavior only affects version 3 of Platform.