Checked your PR and repo, it's not a fork issue, the repo merged into is an original repository and your PR was correctly merged into its main branch. So the most likely cause left is your commit email. Go to the PR's Commits tab and click on your commit. Check if your name/avatar next to the commit is a clickable link to your GitHub profile. If it's shown as plain text instead, that means the email in your local git config doesn't match a verified email on your GitHub account, and unlinked commits don't count toward achievements.

To fix it going forward: run git config user.email and make sure it matches an email listed under Settings → Emails on your account (and is verified). For this specific PR though, since it's already merged, you may need to wait and see if GitHub backfills it, or reach out to GitHub Support if it still hasn't appeared after a few more days.

If your pull request was accepted and merged but you didn't receive the Pull Request badge, check the following:

Make sure the repository is public and the PR was merged into the default branch.
Verify that the email used in your commit matches a verified email on your GitHub account:
git config --global user.email
Open the commit in the merged PR and confirm your GitHub profile/avatar is linked to the commit.
Contribution badges and achievements may take some time to update, so wait 24–72 hours after the merge.