Unicode menu items not displaying

I have an issue where the options for choosing open/close icons are not displaying properly in the general options > advanced options section of the plugin. Instead it only shows square boxes (see image below).

This means the mobile menu is showing the following open/close icons in the front-end menu.

Plugin version is 2.8.4

WordPress version is a 6.2.2

This is currently a development site, so ideally need to fix before we go live. Thanks.

The page I need help with: [log in to see the link]