I got a similar problem using unicode and fontawesome.
When I wrote:
font-family: 'Font Awesome\ 5 Free';
content: "\f061"; /* FontAwesome Unicode */
On Google Chrome, a square appears instead of the icon.
The new version of Font Awesome also requires
font-weight: 900;
That works for me.
From: https://github.com/FortAwesome/Font-Awesome/issues/11946