List Icons on 2017 Theme Startpage will disappear

We use the the Shortcode List Tag on our Startpage, builded with a child theme, based on the 2017 WP Standard Theme. The Startscreen in this Themes is built by Sections, using WP-Pages as a Source. We noticed, that, if we are using Safari (Mac) as Browser, everything is ok, our List Icons (a simple “check” hook) are displayed fine on this start page. If we use Firefox, Chrome or Opera, the Icons ar not visible or has empty aliases. If you create a single test page with same code, they are visible on all Browsers. How could we display our List with these icons on all Browsers?

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