I have set up TinyMCE settings according to http://world.episerver.com/documentation/developer-guides/CMS//Content/Properties/Property-settings/

with the addition of

settings.NonVisualPlugins.Add("DefaultSettings");

and

[TinyMCEPluginNonVisual(
        AlwaysEnabled = true,
        ServerSideOnly = true,
        PlugInName = "DefaultSettings",
        DisplayName = "Init options",
        Description = "Init options for the HTML editor",
        EditorInitConfigurationOptions = @"{
            paste_text_sticky : true,
            paste_text_sticky_default: true,
            init_instance_callback: 'insert_default_content'
        }")]
    public class DefaultSettingsPlugin
    {
    }

now note the init_instance_callback: 'insert_default_content' - https://stackoverflow.com/questions/16508001/how-to-set-tinymce-default-content

when I open a block that has TinyMCE and some text already entered, in that function (insert_default_content) TinyMCE instance will return proper content by calling getContent(). If I switch from edit mode to preview mode, and back to edit mode the same method will return an empty string. Why?