@Irrawaddy welcome to the forum.
For the Joplin devs out there I can sort of confirm this. It looks like the scrollbar does work but the area you have to put your mouse on is a very narrow strip on the left of the scrollbar. The rest of the scrollbar area enables column resize. Almost like it has been reversed from the way one would expect it to be.
Joplin v1.2.6 Win10 2004
Edit: It looks like the column resizer lays over the scrollbar and, at 10px, is a bit too wide.