AMBOSS says starts at 6 months
https://next.amboss.com/us/article/b40H3T?q=stranger%20anxiety#Z95ddba91042a1576b44f67cf208180ae
I agree. All the sources are saying 6 months (Pixorize, First Aid, Amboss, the chart in the additional resources, etc.). The only source that’s saying stranger anxiety develops at 9 months is table in the “Extra” Field (which I assume is from Uworld). I’d suggest the anking deck maintainers consider changing the card to say “6 months” due to the majority of sources stating so, then adding a comment in the “extra” field stating that Uworld says 9 months.