Sticky element disappearing instead of scrolling

Hi,

The sidebar of my post pages (the Table of Contents, like button, etc) was working completely fine with this plugin up until today. I have not installed any new plugins, tried disabling every plugin except this one, and have not made any design changes either.

Instead of scrolling down with you, it just disappears from the screen completely and only returns when you scroll back up above it.

It’s super odd because it literally just happened out of nowhere, and no plugins seem to be conflicting with it.

Any thoughts would be appreciated.

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