Pango does not give the location for certain graphemes

I was writing a program to render text to an image and draw bounding boxes around characters using Pango, Cairo and PangoCairo. I am using the Rust bindings to these libraries called gtk-rs.

After laying out the next I am splitting the text into graphemes using unicode_segmentation and finding the position of these graphemes using index_to_pos which basically translates to pango_layout_index_to_pos. Here is the code I wrote to draw these bounding boxes.

use unicode_segmentation::UnicodeSegmentation;

use crate::ImageDims;

#[derive(Debug)]
pub struct BoundingBox {
    pub x: i32,
    pub y: i32,
    pub height: i32,
    pub width: i32,
    pub akshara: String,
}

pub type BoundingBoxes = Vec<BoundingBox>;

pub fn get_bounding_boxes(layout: pango::Layout, dims: ImageDims) -> BoundingBoxes {
    let mut boxes = BoundingBoxes::new();

    let text = layout.text().unwrap();

    for (idx, graphemes) in text.grapheme_indices(true) {
        let rect = layout.index_to_pos(idx as i32);

        boxes.push(BoundingBox {
            x: rect.x(),
            y: rect.y(),
            height: rect.height(),
            width: rect.width(),
            akshara: graphemes.to_string(),
        });
    }

    // adjust the values for the cairo context
    boxes.iter_mut().for_each(|b| {
        b.x = b.x / pango::SCALE + dims.padding;
        b.y = b.y / pango::SCALE + dims.padding;
        b.width = b.width / pango::SCALE;
        b.height = b.height / pango::SCALE;
    });

    boxes
}

However, in the rendered image some of the characters do not have bounding boxes at all. For example । on the last line or ए in the last word. There are other abnormalities like भी in the third word.

Some characters do not have bounding boxes

How do I fix this?