iOS Vision framework text detection does not detect single digit numbers?

Question

Using text detection inside iOS vision framework struggles to detect single digit numbers, it seems to detect certain digits fine but with others it will fail such as the number 1, 2 or 3 etc. I am just trying to highlight/extract the numbers in this simple graph known as an Audiogram. Audiograms have an X and Y axis which both contain numbers, all I seek to do is read them and add bounding boxes around them.

I have been successful in reading some of the numbers but it just fails on others, why does the vision framework detect the number 0 but fails on 1 or 2? Is there something I can do to remedy this issue?