Incorrect DNS record on R53 pointing to my server

This can happen, as from time to time, customers may set a resource record in a Route53 hosted zone (or any other DNS service for that matter) that points to an IP that belongs to AWS (or another provider). Later on that IP may become used by resources that are assigned to you, and customers may neglect to remove that resource record from their hosted zone, or other DNS service.

Since you have verified that this is coming from a Route53 hosted zone, you don't however provide enough detail to verify this from my side, you would need to reach out to AWS Support and give them the domains, your IP and so on, and they can take it further.

If you are not getting excessive unwanted traffic from this DNS resource record, you can do a layer-7 block on your web-server as well - and just have it send a 403 or a 503 response to requests that contain the host-header of the unwanted DNS resource record name.

What you're describing is unusual, as AWS Route 53 static IPs are dedicated to your account and should not be assigned to websites you don't own. There are a few possible explanations for what you're observing:

1. DNS Misconfiguration: The owner of the other domain may have incorrectly entered your IP address in their DNS records. This could be a simple typo or mistake in their Route 53 configuration.

2. Shared IP Address: If you're using a service that shares IP addresses across multiple customers (like some shared hosting providers), it's possible that the other domain is legitimately configured to use the same IP address.

3. Server Configuration: Your web server might be configured to respond to requests for multiple domain names. Check your web server configuration to see if it's set up to handle requests for domains you don't recognize.

4. DNS Propagation: While you mentioned it's been nearly a day, some DNS changes can take up to 48 hours to fully propagate globally.

If you're concerned about this situation, you have a few options:

• Configure your web server to only respond to domains you own
• Contact the owner of the other domain if possible
• If you believe there's an actual issue with Route 53's configuration, you can contact AWS Support

Since you mentioned the other site shows a Cloudflare error, it's possible they're in the process of changing their configuration, which might resolve itself soon.
Sources
My Lightsail static IP is also assigned to another website which I don't own | AWS re:Post
I changed DNS settings, but they haven't taken effect - Amazon Route 53
DNS resolution error | AWS re:Post