Inconsistent CI context for bottom PR

[ericqu8]

Says I have a workflow that supports both single PR and stack PR. I have dedicated jobs for each type.
However, the bottom PR of the stack always have github.event.pull_request.stack to null when it's first created, hence, it echoes "Single PR"
Subsequent pushes to the bottom PR trigger the workflow again but now, it recognizes the stack therefor it echoes "Stacked PR".

Is this intended?

IMO, the pull_request.stack should have returned NOT null for the bottom PR when it's first created.

on:
  pull_request:
    branches:
      - main

jobs:
  job-single:
    runs-on: ubuntu-latest
    steps:
      - name: Single PR
        if: github.event.pull_request.stack == null
        run: echo "Single PR"

  job-stack:
    runs-on: ubuntu-latest
    steps:
      - name: Stacked PR
        if: github.event.pull_request.stack != null 
        run: echo "Stacked PR"

see https://docs.github.com/en/pull-requests/how-tos/merge-and-close-pull-requests/optimizing-ci-for-stacked-pull-requests