Do not reuse a server constant built in another scope.

src/Analyzer/Resolve/resolveFunction.cpp

@@ -1629,7 +1629,11 @@ ProjectionNames QueryAnalyzer::resolveFunction(QueryTreeNodePtr & node, Identifi
         ///
         /// `getSetting` and `rowNumberInAllBlocks` are non-deterministic but must NOT be shared: the cache
         /// is global across scopes, and e.g. `SETTINGS` can change `getSetting`'s result for every scope.
+        ///
+        /// Server constants (`hostName`, `serverUUID`, ...) capture `Context::isDistributed` when built.
+        /// A local `UNION` branch and a later cluster branch must not share that instance.
         if (function && !function->isDeterministic()
+            && !function->isServerConstant()
             && function_name != "getSetting" && function_name != "rowNumberInAllBlocks")
         {
             auto hash = function_node_ptr->getTreeHash();

tests/queries/0_stateless/05294_union_hostname_with_cluster_function.reference

@@ -0,0 +1,2 @@
+1	1	1
+2	2	1

tests/queries/0_stateless/05294_union_hostname_with_cluster_function.sql

@@ -0,0 +1,15 @@
+-- A local GROUP BY hostName() must not be reused for a later cluster branch.
+-- hostName() is folded on the local branch and must stay a per-shard column on the cluster branch.
+
+SELECT tag, c, h = hostName()
+FROM
+(
+    SELECT hostName() AS h, count() AS c, 1 AS tag
+    FROM numbers(1)
+    GROUP BY h
+    UNION ALL
+    SELECT hostName() AS h, count() AS c, 2 AS tag
+    FROM cluster('test_cluster_two_shards', system.one)
+    GROUP BY h
+)
+ORDER BY tag;