Specifically here:

https://github.com/webbukkit/dynmap/blob/v3.0/forge-1.12.2/src/main/java/org/dynmap/forge_1_12_2/DynmapPlugin.java#L1702

I have a mod that prevents the unloading of worlds, your plug doesn't listen to see if this event is cancelled and is assuming it was; and stops all render processing.

sigh the damn Forge event isn't cancellable.... So I had to go about this another way....