Setting up load balancing for Craft on Laravel Forge
I figured out a solution, though I'm not totally clear why it works.

Adjusting the server statement below

server {
    listen 80;
    listen [::]:443 ssl;
    ...


to read 

server {
    listen 80;
    listen 443 ssl;
    listen [::]:443 ssl;
    ...


Solved my problem and allowed my load balancer to direct to my application servers.