TypedAPI: Fix additionalproperty ser/de

[Anaethelion]

Fixes #830

[github-actions[bot]]

The backport to 8.13 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.13 8.13
# Navigate to the new working tree
cd .worktrees/backport-8.13
# Create a new branch
git switch --create backport-833-to-8.13
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 b57f7467aab9eb46884c3fa33f60461a6b75d99a
# Push it to GitHub
git push --set-upstream origin backport-833-to-8.13
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.13

Then, create a pull request where the base branch is 8.13 and the compare/head branch is backport-833-to-8.13.