Why does custom CSS disappear from the UI & Layout Composer

Hello.  I attempt to add custom CSS snippets to the appropriate spot in the UI & Layout Composer.  For example, I add the following:

.fpd-thumbnail-preview {display: none;}

After it is added I update the page and refresh to see if the changes were applied to the product pages.  However, nothing happens when this is added.  And when I go back to the UI & Layout Composer - Custom CSS tab -- the snippet I added has disappeared.

Are the custom snippets a person adds to their dashboard supposed to remain displayed so that they can be adjusted later?  Or are they meant to disappear like that?  

And why don't the snippets added do anything?  Any help would be appreciated

Thanks