This is well-known in the Moodle development community, but it's still a very important issue: the jump menu is like quicksand for a screen reader user. The jump menu contains a list of every page in the course. Once a page is selected, the user is taken there automatically, with no further action.
This is a problem in a common use case, observed in all screen reader users:
1. User tabs into the control.
2. User scrolls up and down, reading the options.
3. User tabs out, intending to go to the next item on the page.
4. At this point, the last page they read in the list is loaded.
Why it matters
Once a user focuses the control it's very difficult to exit it without switching to another page. Because this takes place without explicit action on the part of the user it is unexpected behavior, and causes frustration and confusion.