Uunless the url contains a "previewPersona" preview attribute, then on load the left and right arrows on the switcher are disabled. As soon as you choose a user from the dropdown, then the arrows are enabled.

Technical notes:
It seems like the switcher does not properly initialize itself. So perhaps this is a UI ticket.

I experimented a little and did find a way to properly initialize the siwtcher.

in PagePreviewSubApp#start, after the check

previewLocation.hasAdditionalParameters()

To add something along the lines of this in order to add a default for the previewItem:

if (null == previewItem.getItemProperty("persona")){
            final Property<String> property = new DefaultProperty<String>(String.class, "/");
            previewItem.addItemProperty("persona",property);
        }

But I guess you would want to query the config for the proper property instead of hardcoding "persona".