diff --git a/extensions/cornerstone-dicom-seg/src/viewports/OHIFCornerstoneSEGViewport.tsx b/extensions/cornerstone-dicom-seg/src/viewports/OHIFCornerstoneSEGViewport.tsx
index 4cc6edcc38c..10377831a36 100644
--- a/extensions/cornerstone-dicom-seg/src/viewports/OHIFCornerstoneSEGViewport.tsx
+++ b/extensions/cornerstone-dicom-seg/src/viewports/OHIFCornerstoneSEGViewport.tsx
@@ -292,10 +292,12 @@ function _getReferencedDisplaySetMetadata(referencedDisplaySet, segDisplaySet) {
 
   const { PixelMeasuresSequence } = SharedFunctionalGroup;
 
-  const PixelMeasures = Array.isArray(PixelMeasuresSequence)
-    ? PixelMeasuresSequence[0]
-    : PixelMeasuresSequence;
+  const PixelMeasures =
+    (Array.isArray(PixelMeasuresSequence) ? PixelMeasuresSequence[0] : PixelMeasuresSequence) || {};
 
+  if (!PixelMeasuresSequence || Object.keys(PixelMeasures).length === 0) {
+    console.warn('PixelMeasuresSequence missing from SEG instance metadata.');
+  }
   const { SpacingBetweenSlices, SliceThickness } = PixelMeasures;
 
   const image0 = referencedDisplaySet.images[0];
diff --git a/platform/core/src/classes/MetadataProvider.ts b/platform/core/src/classes/MetadataProvider.ts
index 1b8d4308534..1f4bfff8bb2 100644
--- a/platform/core/src/classes/MetadataProvider.ts
+++ b/platform/core/src/classes/MetadataProvider.ts
@@ -60,7 +60,13 @@ class MetadataProvider {
       return;
     }
 
-    return (frameNumber && combineFrameInstance(frameNumber, instance)) || instance;
+    const combined = frameNumber && combineFrameInstance(frameNumber, instance);
+    if (combined) {
+      // Add imageId to multiframe result so it matches single-frame instance.
+      combined.imageId = imageId;
+      return combined;
+    }
+    return instance;
   }
 
   get(query, imageId, options = { fallback: false }) {
diff --git a/platform/core/src/utils/combineFrameInstance.ts b/platform/core/src/utils/combineFrameInstance.ts
index 657619d0053..bcce4658abf 100644
--- a/platform/core/src/utils/combineFrameInstance.ts
+++ b/platform/core/src/utils/combineFrameInstance.ts
@@ -20,7 +20,9 @@ const combineFrameInstance = (frame, instance) => {
   } = instance;
 
   if (NumberOfFrames < 2) {
-    return instance;
+    // Return a fresh proxy so callers can set properties (eg. imageId)
+    // without mutating the shared `instance` stored in DicomMetadataStore.
+    return Object.create(instance);
   }
 
   instance.ImageType = dicomSplit(ImageType);
@@ -32,7 +34,8 @@ const combineFrameInstance = (frame, instance) => {
 
   if (
     (PerFrameFunctionalGroupsSequence && SharedFunctionalGroupsSequence) ||
-    hasDetectorButMissingSpatialInfo || NumberOfFrames > 1
+    hasDetectorButMissingSpatialInfo ||
+    NumberOfFrames > 1
   ) {
     // this is to fix NM multiframe datasets with position and orientation
     // information inside DetectorInformationSequence
@@ -160,37 +163,37 @@ const combineFrameInstance = (frame, instance) => {
  * The storage key in the parent is in key
  */
 function createCombinedValue(parent, functionalGroups, key) {
-  if (parent[key]) {
-    return parent[key];
-  }
-  // Exclude any proxying values
-  const newInstance = Object.create(parent);
-  Object.defineProperty(parent, key, {
-    value: newInstance,
-    writable: false,
-    enumerable: false,
-  });
-  if (!functionalGroups) {
-    return newInstance;
-  }
-  const shared = functionalGroups
-    ? Object.values(functionalGroups)
+  // Ensure there's a cached template on the parent (a non-writable, non-enumerable object)
+  if (!parent[key]) {
+    const templateInstance = Object.create(parent);
+    Object.defineProperty(parent, key, {
+      value: templateInstance,
+      writable: false,
+      enumerable: false,
+    });
+
+    if (functionalGroups) {
+      const shared = Object.values(functionalGroups)
         .filter(Boolean)
         .map(it => it[0])
-        .filter(it => typeof it === 'object')
-    : [];
-
-  // merge the shared first then the per frame to override
-  [...shared].forEach(item => {
-    if (item.SOPInstanceUID) {
-      // This sub-item is a previous value information item, so don't merge it
-      return;
+        .filter(it => typeof it === 'object');
+
+      // merge the shared/per-frame attributes onto the template
+      [...shared].forEach(item => {
+        if (item.SOPInstanceUID) {
+          // This sub-item is a previous value information item, so don't merge it
+          return;
+        }
+        Object.entries(item).forEach(([k, value]) => {
+          templateInstance[k] = value;
+        });
+      });
     }
-    Object.entries(item).forEach(([key, value]) => {
-      newInstance[key] = value;
-    });
-  });
-  return newInstance;
+  }
+
+  // Return a fresh object that inherits from the cached template so
+  // mutations by the caller do not affect the cached template.
+  return Object.create(parent[key]);
 }
 
 export default combineFrameInstance;