The Default Thumbs: ON setting(in Designs>Desktop Layout>Thumbnails) only displays the thumbnails first, if the gallery is selected from the menu list on your website by the visitor, this is because selecting it from the Menu is what activates that thumbnail-first style display.
Wondering if that applies to you?
Check to see if your gallery in question, is hidden in your menu.
Indicated by an eye-ball icon that is greyed out in the Website Menu column of your Media Library.
Are you trying to send that hidden gallery to someone?
If so, then you must manually append: /thumbs to the end of the HIDDEN URL before you send the link to someone.
Example: If the HIDDEN URL: that you're copying to send to someone is: `https://designx.rocks/OVERVIEW/Gallery-III/1`
Copy it from the EDIT - SETTINGS panel.
Then in your email or in an open text document paste that link. Then please delete the /1 at the end of the url and append: /thumbs
So it looks like this: `https://designx.rocks/OVERVIEW/Gallery-III/thumbs` before you copy+paste and send the link to someone.
(*Your URL will be specific to your public domain and menu titles)
NOTE: If the gallery is live or your change the VISIBILITY setting to ON, then anyone selecting that gallery from the menu will see the thumbs first, because the menu selection is what activates the thumbs to display by default (thumbs-first)