Application network traffic filtered, when screen is locked? I observe that network traffic for applications seems to be blocked, or perhaps queued, when the screen is locked, on my MacOS High Sierra 10.13.4 Mac Mini.

The most obvious example I have seen: when running the native MacOS Spotify client, and unlocking the screen after being away for a few days, the client becomes animated - it shows every song I've played on other clients while I've been away from the Mac console, at a high rate, flipping from one to the next, one per second, or so.

This leads me to guess that the Mac caches that traffic, until the screen is unlocked? 

What is happening, here?  

How can I disable this behavior, and allow the apps I'm running to get network traffic when the screen is locked?
You are confusing "network traffic filtering" with Application "synchronization."

When your Mac goes to sleep, it suspends all other applications from execution.  Meaning, your Spotify client doesn't get updated.  When you use your Spotify client on your mobile device, for example, the Spotify service (in the cloud) remembers all of that activity.  When you wake your client, it checks in with the service, looks for changes and then synchronizes them.  This is the case for any application that syncs it's settings via the cloud (Safari, Messages, FaceTime, Photos, Spotify, FireFox, Edge Browser, etc.)

Network Traffic Filtering looks at individual packets and either blocks them or allows them based on a "filter" rule.  It won't queue them up (and especially not on a sleeping computer) for later processing.