Application load balancer fails to upload large file

The problem I'm seeing is that each event appears to be coming from the load balancer IP and not from the EC2 instance IP. I'm convinced that when the load balancer forwards traffic, it puts its IP on the packet.

You're convinced of this because yes, that is precisely how load balancers are designed to work.

Is there a setting in AWS on the load balancer that I can change to forward packets with the actual IP address instead of the load balancers IP?

Think that through for a minute. Sure, an LB could theoretically change the IP headers so that the source IP matches the backend server. There are several problems with this, though:

1. Your backend servers have private IPs. Which your clients aren't going to be able to hit.
2. When your clients hit your ELB, they first have to stand up a TCP connection before any HTTP can happen. So they have a TCP connection with the ELB, not with the backend server. Therefore when the client sees a packet with a source IP that doesn't match a current connection, it's going to discard it.
3. You likely don't want to do this anyway, as backend instances change frequently and your clients shouldn't need to worry about that, nor should they be able to discover your private infrastructure.

Why do you want this anyway? If you really want your clients to be able to discover this information, you can possibly put the private IP into an HTTP response header, which your client has access to.

We cannot know the reason why they don't have it. We cannot know AWS design / implementation decisions which cause the behaviour. Only people who work with Amazon on this feature know, and they are most likely under NDA.

Your only chance to get a valid answer for this is to ask AWS.