When courses are created for an upcoming semester, the current semester's courses may be pushed from the dashboard if the total course number for the user is over 20.
Canvas regulates the number of courses presented on the dashboard to a maximum of 20, but you can locate missing courses under "Courses > All Courses".
When no courses are favorited, the courses list automatically displays up to 20 courses alphabetically in the drop-down menu.
However, once you have selected at least one course as a favorite, only your favorite courses will appear in the Courses list.
Please review this article about Customizing my Courses List for more information.