Hey,

adding to SPIGOT-2303 there is another problem with using cunkproviderserver.getLoadedChunkAt(). The main problem again is the newly introduced boolean in chunk.java to keep chunks from being unloaded.
It is used in cunkproviderserver.getLoadedChunkAt()

I wrote a less spiking processChunkGC and this problem came up
In CraftWorld.java the method
unloadChunk0(int x, int z, boolean save, boolean safe)
The setNeighborUnloaded-Code would keep chunks from unloading just because its neighbor got unloaded.

    private boolean unloadChunk0(int x, int z, boolean save, boolean safe) {
        [...]
                net.minecraft.server.Chunk neighbor = world.getChunkProviderServer().getLoadedChunkAt(chunk.locX + x, chunk.locZ + z); 
                if (neighbor != null) {
                    neighbor.setNeighborUnloaded(-xx, -zz);
                    chunk.setNeighborUnloaded(xx, zz);
                }
 [...]
    }

Changing this from getLoadedChunkAt to getChunkIfLoaded should fix the problem