This is something that happened since the intro of FA, and should be fixed. The code in stable 32 (https://github.com/moodle/moodle/blob/MOODLE_32_STABLE/course/renderer.php#L534) used to use an input and set title to $imgtitle, but now uses a button and wraps a pix_icon. In effect, the user sees no tooltip for the manual completion icon in either state (complete/not complete). We might be able to add a title to the button? Needs a look in any case.