Critical: 'What is my IP Address' Tool Suddenly Displays Internal IPs, Public IP Fetch Failing!

We run a simple but critical web tool, "What is my IP Address", which normally just shows the user's current public IP. For the past few hours, it's suddenly broken, causing a major headache and completely disrupting our core utility.

Instead of displaying the correct external/public IP address, our tool is consistently returning internal network IPs (like 192.168.x.x or 10.x.x.x) or, worse, just failing to fetch anything at all, showing an empty string. This is happening across multiple client networks and browsers, making our service effectively unusable for its primary purpose of accurate public IP detection.

I've been trying to fix this for hours and I'm completely stuck. Here's what I've attempted so far:

• Checked server logs for any recent configuration changes or errors that might explain this sudden shift.
• Restarted the web server (nginx/apache) and the underlying application service multiple times, hoping for a quick fix.
• Verified firewall rules to ensure no new blocks on outbound connections to common IP lookup services. Everything seems to be open as usual.
• Tested direct cURL requests from the server to external IP lookup APIs (e.g., icanhazip.com, ipify.org) – these commands return the correct public IP from the server's perspective, which is incredibly confusing!
• Inspected the application code (PHP/Python backend) that processes the request and calls external services. No recent code changes were deployed, so it shouldn't be a code-level bug we introduced.
• Checked DNS resolution from the server – seems fine, no issues resolving external domains.
• Tried different network environments for testing the tool (my home network, a mobile hotspot, a friend's office network) – same incorrect internal IP or blank result every single time.

Here's a typical output I'm seeing from the tool:

Your Public IP Address: 192.168.1.10
// Or sometimes:
Your Public IP Address: 

// Expected accurate public IP detection result:
// Your Public IP Address: 203.0.113.45 (user's actual public IP)

What could possibly be causing our tool to incorrectly identify and display internal network addresses to users, or just fail completely, when direct server-side calls work perfectly? Is there some obscure proxy configuration, a specific server header issue, or even a client-side caching problem I'm completely missing that would trick the application into seeing an internal IP instead of the user's actual public IP? Any insights would be incredibly helpful as we're completely stuck and this is a core utility for many users.