If a user with a large number of courses (> 25) browses to their (default, unedited) "/my" page and then goes back to the homepage or a course page, their "My Courses" block is truncated to about 23 courses. This is completely reproducible, but only apparent where a user has a large number of courses.

What seems to be happening (as far as I can tell), is that the default "/my" page calls

$courses = get_my_courses($USER->id, 'visible DESC,sortorder ASC', '*', false, 21);

to display the course overview with a limited number of courses. It seems this list then gets cached in $courseids and the subsequent sql query run to generate the "my courses" block includes a line:

WHERE c.id IN ($courseids);

where $courseids seems to be much the same list as was displayed on /my.

Apologies to not have completely worked this out as yet. I'll have a further look when time permits.