Existing contents get deleted on Live Preview when adding new content on a matrix field When I add a new content for a matrix block field on an existing entry, all its previous contents gets deleted from the Live Preview, except for the editor which still has the previous contents. And then, when I try to save the entry, it seems that what is shown in the Live Preview was the only one saved and the previous data were gone.
Do you have this problem in a multisite-installation? If so, it could be something with your propagation mode on the matrix field.

Please give us some more details to your setup: which version of Craft CMS, which propagation mode, multisite or not.