Well, this is embarassing.

proxy/src/main/java/com/velocitypowered/proxy/network/netty/SeparatePoolInetNameResolver.java

@@ -2,6 +2,7 @@ package com.velocitypowered.proxy.network.netty;
 
 import com.google.common.cache.Cache;
 import com.google.common.cache.CacheBuilder;
+import com.google.common.collect.ImmutableList;
 import com.google.common.util.concurrent.ThreadFactoryBuilder;
 import io.netty.resolver.AddressResolver;
 import io.netty.resolver.AddressResolverGroup;
@@ -56,7 +57,7 @@ public final class SeparatePoolInetNameResolver extends InetNameResolver {
       resolveExecutor.execute(() -> {
         promise.addListener(future -> {
           if (future.isSuccess()) {
-            cache.put(inetHost, (List<InetAddress>) future.getNow());
+            cache.put(inetHost, ImmutableList.of((InetAddress) future.getNow()));
           }
         });
         this.delegate.resolve(inetHost, promise);