Ok, then the bug is that top/bottom doesn't move just the active cursor.
Repro
Select block
Cursor down several
add-multiple-cursors
Top-of-buffer
Now, it sets several bookmarks - one for each cursor, with only a single cursor at the top of the buffer.
If you move the cursors at all before top-of-buffer then it doesn't do this.
Another repro:
Set multiple cursors with find.
Top-of-buffer
Same thing.
Also, moving the cursors at all before top-of-buffer avoids the problem.