NSAttributedString size calculation returns different results with .usesFontLeading option

TLDR; I found out that .usesFontLeading return the different result in iOS 13, but I want to know why ?

I have a bug in iOS 13 that my app shows the wrong text height. This bug does not happen in iOS 12.

I have the code to calculate the multi-line height of NSAttributedString.

I found that with different options, one has font leading, another one does not. The code returns different results.

let height_with_font_leading = text.boundingRect(with: constraintSize,
                                                 options: [.usesLineFragmentOrigin, .usesFontLeading],
                                                 context: nil).height

let height_with_no_font_leading = text.boundingRect(with: constraintSize,
                                                    options: [.usesLineFragmentOrigin],
                                                    context: nil).height

The height from the calculations are shown as images below.

As you see, the height without .usesFontLeading is correct, because the label can show full text without truncation.

1. What is the purpose of .usesFontLeading ?
2. When should I use it ?

Here is the full snippet of my code:

https://gist.github.com/nRewik/7760ac31f107642c70eb9e659862ac3b