diff --git a/weed/s3api/auth_account_collapse_test.go b/weed/s3api/auth_account_collapse_test.go
index cebca51fe..49380e33d 100644
--- a/weed/s3api/auth_account_collapse_test.go
+++ b/weed/s3api/auth_account_collapse_test.go
@@ -70,3 +70,62 @@ func TestCheckAccessByOwnershipDeniesNonOwner(t *testing.T) {
 	owner.Header.Set(s3_constants.AmzAccountId, AccountAdmin.Id)
 	assert.Equal(t, s3err.ErrNone, s3a.checkAccessByOwnership(owner, "b"), "the actual owner is still allowed")
 }
+
+// An account-less identity's synthesized account must be registered in the
+// account lookup so its id resolves to a display name. Otherwise ACL grantee
+// validation and owner display report the id as "not exists" — the regression
+// where a canned PutObjectAcl granting to the caller's own account returned
+// 400 InvalidRequest.
+func TestUnscopedIdentityAccountResolvesByName(t *testing.T) {
+	resetMemoryStore()
+
+	config := `{
+  "identities": [
+    {"name": "alice", "credentials": [{"accessKey": "alice_ak", "secretKey": "alice_sk"}], "actions": ["Read", "Write"]}
+  ]
+}`
+	tmp, err := os.CreateTemp("", "s3-config-*.json")
+	require.NoError(t, err)
+	defer os.Remove(tmp.Name())
+	_, err = tmp.WriteString(config)
+	require.NoError(t, err)
+	require.NoError(t, tmp.Close())
+
+	iam := NewIdentityAccessManagementWithStore(&S3ApiServerOption{Config: tmp.Name()}, nil, "memory")
+
+	assert.Equal(t, "alice", iam.GetAccountNameById("alice"),
+		"account-less identity id must resolve to a display name for ACL/owner validation")
+}
+
+// When an account is explicitly configured with the same id an account-less
+// identity would synthesize, the identity must reuse that configured account so
+// its custom display name/email are preserved.
+func TestUnscopedIdentityReusesConfiguredAccount(t *testing.T) {
+	resetMemoryStore()
+
+	config := `{
+  "accounts": [
+    {"id": "alice", "displayName": "Alice Smith", "emailAddress": "alice@example.com"}
+  ],
+  "identities": [
+    {"name": "alice", "credentials": [{"accessKey": "alice_ak", "secretKey": "alice_sk"}], "actions": ["Read"]}
+  ]
+}`
+	tmp, err := os.CreateTemp("", "s3-config-*.json")
+	require.NoError(t, err)
+	defer os.Remove(tmp.Name())
+	_, err = tmp.WriteString(config)
+	require.NoError(t, err)
+	require.NoError(t, tmp.Close())
+
+	iam := NewIdentityAccessManagementWithStore(&S3ApiServerOption{Config: tmp.Name()}, nil, "memory")
+
+	assert.Equal(t, "Alice Smith", iam.GetAccountNameById("alice"),
+		"explicitly configured account display name must be preserved")
+
+	alice, _, found := iam.LookupByAccessKey("alice_ak")
+	require.True(t, found)
+	require.NotNil(t, alice.Account)
+	assert.Equal(t, "Alice Smith", alice.Account.DisplayName,
+		"identity must reuse the configured account, not the synthesized one")
+}
diff --git a/weed/s3api/auth_credentials.go b/weed/s3api/auth_credentials.go
index f5fb56efb..1a11b490c 100644
--- a/weed/s3api/auth_credentials.go
+++ b/weed/s3api/auth_credentials.go
@@ -627,7 +627,18 @@ func (iam *IdentityAccessManagement) ReplaceS3ApiConfiguration(config *iam_pb.S3
 			t.Account = &AccountAnonymous
 			identityAnonymous = t
 		case ident.Account == nil:
-			t.Account = accountForUnscopedIdentity(t.Name)
+			// Account-less identities own resources under a distinct id derived
+			// from their name. Reuse an explicitly-configured account with that
+			// id if one exists (preserving its display name/email); otherwise
+			// synthesize one and register it so the id resolves via
+			// GetAccountNameById (ACL grantee validation, owner display).
+			synthesized := accountForUnscopedIdentity(t.Name)
+			if existing, ok := accounts[synthesized.Id]; ok {
+				t.Account = existing
+			} else {
+				t.Account = synthesized
+				accounts[synthesized.Id] = synthesized
+			}
 		default:
 			if account, ok := accounts[ident.Account.Id]; ok {
 				t.Account = account
@@ -846,7 +857,18 @@ func (iam *IdentityAccessManagement) MergeS3ApiConfiguration(config *iam_pb.S3Ap
 			t.Account = &AccountAnonymous
 			identityAnonymous = t
 		case ident.Account == nil:
-			t.Account = accountForUnscopedIdentity(t.Name)
+			// Account-less identities own resources under a distinct id derived
+			// from their name. Reuse an explicitly-configured account with that
+			// id if one exists (preserving its display name/email); otherwise
+			// synthesize one and register it so the id resolves via
+			// GetAccountNameById (ACL grantee validation, owner display).
+			synthesized := accountForUnscopedIdentity(t.Name)
+			if existing, ok := accounts[synthesized.Id]; ok {
+				t.Account = existing
+			} else {
+				t.Account = synthesized
+				accounts[synthesized.Id] = synthesized
+			}
 		default:
 			if account, ok := accounts[ident.Account.Id]; ok {
 				t.Account = account