Both arrows in the Bookmarks list scroll the bookmarks list down.
I have a lot of bookmarks. Sometimes I move them around. If I grab a bookmark towards the bottom of the list and try to move it up in the list, the list will scroll down instead of up. If I’m at the bottom of the list nothing happens.
You need a list of bookmarks long enough to scroll down a ways in the list. Click on a bookmark near the bottom (hold left mouse button on it) and drag the bookmark to the up arrow in the list. The list will scroll down - unless you are already at the bottom. However, it works properly in reverse - you can move a bookmark from the top down in the list by holding it on the down arrow.
Holding the bookmark on the up arrow should cause the bookmark list to scroll upwards instead of scrolling downwards. The down arrow is working properly. Then you can place the bookmark by releasing the left mouse button.