diff --git a/src/Backend/SolutionErp.Application/PurchaseEvaluations/PurchaseEvaluationFeatures.cs b/src/Backend/SolutionErp.Application/PurchaseEvaluations/PurchaseEvaluationFeatures.cs
index da162e5..7a850e3 100644
--- a/src/Backend/SolutionErp.Application/PurchaseEvaluations/PurchaseEvaluationFeatures.cs
+++ b/src/Backend/SolutionErp.Application/PurchaseEvaluations/PurchaseEvaluationFeatures.cs
@@ -279,7 +279,11 @@ public class UpdatePurchaseEvaluationDraftCommandHandler(
         entity.MoTa = request.MoTa;
         entity.HoSoLink = request.HoSoLink;                         // [HoSoLink] absolute-set như MoTa/DiaDiem (Section 1 text field, null = clear)
         entity.PaymentTerms = request.PaymentTerms;
-        entity.ApprovalWorkflowId = request.ApprovalWorkflowId;     // Mig 23 — User đổi quy trình
+        // [S85 anh Kiet ROOT FIX bug-class S42] null-safe: form Sua KHONG gui approvalWorkflowId
+        // (tao co WF -> bam Sua = MAT WF -> ket submit "chua chon quy trinh" = root-cause whack-a-mole).
+        // Client khong gui field -> GIU WF cu (giong BudgetPeriodAmount duoi); doi WF thi FE phai gui gia tri.
+        if (request.ApprovalWorkflowId is not null)
+            entity.ApprovalWorkflowId = request.ApprovalWorkflowId;     // Mig 23 — User đổi quy trình
         // [S61] null-safe 2 field ngân sách mới (mirror WorkItemId bên dưới):
         // client không gửi → GIỮ giá trị cũ, KHÔNG null-hóa (bug-class S42).
         if (request.BudgetPeriodAmount is not null)