CSS work around so all Font Awesome icons show nicely. Following CSS added to my styles.css for id 'comments' so now icons show properly on all pages even though I have both Font Awesome 5 and 6 now loading on these pages with comments, this is a quick easy fix, and now all my pages show all icons correctly. I also have the Settings > "Styles and Colors" > "Load Font Awesome css lib" is enabled.

/* needed for wpdiscuz */
#comments .fas {
font-family: 'Font Awesome 5 Free';
}

See sample: https://www.ourfingertips.com/sample-comments-with-notifications/