I found that quests that have longer summaries (with a scrollbar) have a tendency to snap back to the top before you’re able to reach the end (about 70-90%-ish).
Here is a quick video showing this in action.
In the video I tried both with a controller and with mouse and both snap back to the top.
With controller it snaps “gracefully”, while using the mouse it flickers between the top position and the current position.
I forgot to add in the initial post that scrolling with the mouse wheel also snaps it to the top or the bottom, there seem to be no intermediate position.
The following clip is from just after the clip in the original post.
In the video I’m slowly scrolling up/down with my mouse but as you can see it just snaps completely to either extremity. You can also see that for somewhat reason the scrollbar itself seem to change size.