Try something

Author: jack-williams

src/compiler/checker.ts

@@ -19004,8 +19004,8 @@ namespace ts {
                 return key = getFlowCacheKey(reference, declaredType, initialType, flowContainer);
             }
 
-            function captureContainingUnion(flow: FlowNode, type: FlowType) {
-                if (flow.flags & (FlowFlags.Assignment | FlowFlags.Condition) && !isIncomplete(type) && type.flags & TypeFlags.Union) {
+            function captureContainingUnion(type: FlowType) {
+                if (!isIncomplete(type) && type.flags & TypeFlags.Union) {
                     containingUnion = type as UnionType;
                 }
             }
@@ -19029,7 +19029,7 @@ namespace ts {
                         for (let i = sharedFlowStart; i < sharedFlowCount; i++) {
                             if (sharedFlowNodes[i] === flow) {
                                 flowDepth--;
-                                captureContainingUnion(flow, sharedFlowTypes[i]);
+                                captureContainingUnion(sharedFlowTypes[i]);
                                 return sharedFlowTypes[i];
                             }
                         }
@@ -19107,7 +19107,7 @@ namespace ts {
                         sharedFlowTypes[sharedFlowCount] = type;
                         sharedFlowCount++;
                     }
-                    captureContainingUnion(flow, type);
+                    captureContainingUnion(type);
                     flowDepth--;
                     return type;
                 }