Post with long list of comments - Scroll Thumb disappears

On a desktop, the scrollbar scrolls completely out of view (way passed the scrollbar track) before even reaching the end of the comments. You must use scroll wheel on mouse to continue and to bring scroll thumb back into view.

To see in action, go to: https://community.wpmanageninja.com/portal/space/community-meta/post/introducing-fluentcommunity

Once modal opens, scroll down as if you are reading the comments. Scrollbar thumb is gone WELL before end of comments.