Bug: icon does not appear
The button that allow to open the Table of Contents panel contains an icon. By default, an icon is defined but it is possible to specify an other other included into the Fluent UI solution: https://developer.microsoft.com/en-us/fluentui#/styles/web/icons
The icon does not appear. Even if the you keep the default icon or specify a new one that's included into the Fluent UI, the icon still not appearing.
This behavior seems linked with the environment of SharePoint online but it is currently not possible to determine the point that occur this issue.
Whatever it is the icon (the default one or a specified one), it should be visible into the button.