diff --git a/src/measurements/MeasurementContainer.js b/src/measurements/MeasurementContainer.js
index 9ceb8af..5788f3d 100644
--- a/src/measurements/MeasurementContainer.js
+++ b/src/measurements/MeasurementContainer.js
@@ -425,16 +425,34 @@ class MeasurementContainer {
       // Legacy single measurement
       if (posBucket?.getCurrentValue) return posBucket;
 
-      // Child-aware: pick requested child, otherwise fall back to default, otherwise first available
+      // Child-aware lookup. Two separate sources of "child-id" on the
+      // container, with DIFFERENT strictness:
+      //
+      //   _currentChildId : transient, set by .child(name) inside a chain.
+      //                     Explicit per-call. STRICT — if the named child
+      //                     does not exist, return null. Silent fall-through
+      //                     to a sibling would mask a missing-stream read
+      //                     as a wrong-stream read (see pumpingStation
+      //                     spillPrev bug, 2026-05-06).
+      //
+      //   this.childId    : persistent, set by setChildId(id). HINT only —
+      //                     try it first, then fall back to 'default' then
+      //                     first available. Containers registered with a
+      //                     persistent id (rotatingMachine, etc.) write
+      //                     under composed child ids (e.g. 'up-<id>') that
+      //                     don't equal the persistent id, and reads must
+      //                     still resolve to those writes.
       if (posBucket && typeof posBucket === 'object') {
-        const requestedKey = this._currentChildId || this.childId;
         const keys = Object.keys(posBucket);
         if (!keys.length) return null;
-        const measurement =
-          (requestedKey && posBucket[requestedKey]) ||
-          posBucket.default ||
-          posBucket[keys[0]];
-        return measurement || null;
+
+        if (this._currentChildId) {
+          return posBucket[this._currentChildId] || null;
+        }
+        return (this.childId && posBucket[this.childId]) ||
+               posBucket.default ||
+               posBucket[keys[0]] ||
+               null;
       }
 
       return null;