Why would S3 bucket connection require EC2 access when connecting from a remote app - amazon-s3

Question

I have an .NET app that upload files to S3 bucket. The app hosted in a server in a closed network, so we created firewall rule to whitelist outbound traffic to S3(AP-Southeast-2) IP ranges. We noticed that there are intermittent connection issues and throwing following exception;
Amazon.Runtime.AmazonServiceException: A WebException with status SendFailure was thrown. ---> System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. ---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
--- End of inner exception stack trace ---
at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
at System.Net.FixedSizeReader.ReadPacket(Byte[] buffer, Int32 offset, Int32 count)
at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
We no longer get intermittent connection issue when we allow all outbound traffic. But we noticed that there are outbound traffic to EC2 IP range as well.
Question: Why would S3 bucket connection require EC2 access when connecting from a remote app?