Add note about Facebook's mismatched IP address warning

src/connections/destinations/catalog/facebook-pixel/index.md

@@ -247,3 +247,14 @@ Segment does not handle nested properties that need to be blocklisted, including
 
 
 {% include content/client-side-script-unverified.md %}
+
+### Why am I seeing a "Mismatched IP Address" warning in Facebook after enabling both Facebook Conversions API and Facebook Pixel?
+
+When both Facebook Pixel and Facebook Conversions API are enabled, you may see a "Mismatched IP Address" warning in Facebook reports.  This happens because the two sources may send different IP versions (IPv4 vs. IPv6) for the same event:
+
+- Facebook Pixel collects the user’s IP address directly from the browser, [including IPv6 addresses when available](https://developers.facebook.com/docs/marketing-api/conversions-api/parameters/customer-information-parameters#){:target="_blank"}. This happens independently of Segment. Even though Segment’s Analytics.js defaults to collecting only IPv4, the Facebook Pixel automatically collects and sends IPv6 if it's available.
+- Facebook Conversions API sends events to Facebook using data collected by Segment, which typically includes only an IPv4 address.
+
+Since the IP addresses from these two sources don’t always match, Facebook may flag the event with a "Mismatched IP Address" warning.
+
+To resolve this, you can manually collect and send the IPv6 address (when available) in your event payload and send it to Segment. Then, map this data to the Facebook Conversions API destination. This ensures that Facebook receives the same IP version from both sources, preventing mismatches.