WorldBorder is loading chunks through the forceLoad api, and not the paper api. For example:
https://github.com/Brettflan/WorldBorder/blob/master/src/main/java/com/wimbli/WorldBorder/WorldFillTask.java#L326

This is because, internally, force load api will load the target chunk. A fix is to delay adding the force loaded tag until after the chunk is loaded, which can be done via the thenAccept method on the future returned by paper api.

There's no rush to resolve this issue given over at paper we actually have not implemented asynchronous chunk loading for 1.14, so currently it makes no difference.

Ah, so World.setChunkForceLoaded actually loads the chunk rather than just setting a flag to prevent it from being unloaded? I didn't seen an indication of that in the documentation.
https://hub.spigotmc.org/javadocs/spigot/org/bukkit/World.html#setChunkForceLoaded-int-int-boolean-

Do you know of another way to set that flag without also loading the chunk?

No, the flag must be set after loading the chunk

Since our API returns a completablefuture, you can use the thenAccept method to set the force loaded flag - alternatively the plugin ticket api I added to spigot recently.

I don't have a whole lot of time recently, and I'm also not the one who implemented the Paper API into WorldBorder. I took a look briefly and couldn't see how to do that; could you possibly provide a brief example?

Yes, here's an example:

  	PaperLib.getChunkAtAsync(world, x, z, false).thenAccept((Chunk chunk) -> {
  		if (chunk != null) {
  			// if generated
  			world.setChunkForceLoaded(x, z, true);
  			
  			// alternatively for 1.14.4+
  			//world.addPluginChunkTicket(x, z, pluginInstance);
  		}
  	});

Great, thanks.

Based on that code, with my limited knowledge of CompletableFuture, I'd appreciate if you could check this to be sure it looks correct:
f88ede3

Looks good to me

Cool, thanks again.