I believe the problem is in _emit_path_boxes function that is supposed to return a status of operation, but has the following piece of code:
    if (! _cairo_path_fixed_iter_at_end (&iter)) {
	_cairo_boxes_fini (&boxes);
	return FALSE;
    }
When this return is hit, the caller assumes _emit_path_boxes succeeds and the path is recorded incorrectly as an empty rectangle list. This can be reproduced by recording a path 37.1,34 28,34 28,106.4 85.9,106.4 85.9,34 37.1,34.