At the start of the year, as student schedules are being ironed out, students will periodically move from one section of a course to another, or even from one teacher to another. When this happens, the automatic enrollment syncing with FAWeb causes the student to disappear from their old class and re-appear in their new class. Cool!
Except… their grades in their old section also disappear. Panic!
In fact, nothing is ever deleted in Canvas. The grades that a student receives in a particular course are specific the assignments in that course (even if there are seemingly identical assignments in another course). This means that there is no automatic way to transfer grades from one course to another. (If sections have been merged into a single course, this problem does not present itself: that student is still within the umbrella of the same original course.)
What to do
Contact BrianLester to temporarily add the student back into his or her old section, so that you can manually transfer grades from one course to another.
(IInstructure, the developers of Canvas, have intentionally not supported this capability in Canvas, as their research suggests that it is often difficult to mechanically compute comparable grades between different sections.)