diff --git a/CHANGELOG.md b/CHANGELOG.md
index d00feb06..997e5a1c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 
+- Fix nested array stored in DB for readonly dropdown
 - Fix migration abort when a GenericObject container name produces a table name exceeding MySQL's 64-character limit after conversion to GlpiCustomAsset.
 - Fix empty dropdown value (-1) on form submission
 - Fix text area fields size and alignment
diff --git a/inc/container.class.php b/inc/container.class.php
index bd663d6f..c3956d33 100644
--- a/inc/container.class.php
+++ b/inc/container.class.php
@@ -1413,11 +1413,12 @@ public function updateFieldsValues($data, $itemtype, $massiveaction = false)
                     // Add new values to existing ones
                     $existing_values = json_decode($obj->fields[$field_name] ?? '[]', true);
                     $new_values      = is_array($data[$field_name]) ? $data[$field_name] : [$data[$field_name]];
+                    $new_values      = $this->flattenScalars($new_values);
                     $data[$field_name] = json_encode(array_values(array_unique(array_merge($existing_values, $new_values))));
 
                 } else {
                     $value = $data[$field_name];
-                    $value = is_array($value) ? $value : [];
+                    $value = is_array($value) ? $this->flattenScalars($value) : [];
                     $data[$field_name] = json_encode($value);
                 }
             } elseif (array_key_exists('_' . $field_name . '_defined', $data)) {
@@ -1966,6 +1967,22 @@ public static function preItem(CommonDBTM $item)
         return false;
     }
 
+    /**
+     * Flatten a one-level-deep array of mixed scalars/arrays into a flat array of scalars.
+     *
+     * @param array<mixed, mixed> $values
+     * @return array<mixed, mixed> $values the flattened array with only scalar values
+     */
+    private function flattenScalars(array $values): array
+    {
+        $flat = [];
+        foreach ($values as $v) {
+            array_push($flat, ...(is_array($v) ? $v : [$v]));
+        }
+
+        return array_values(array_filter($flat, is_scalar(...)));
+    }
+
     /**
      * Populates fields data from item
      *
diff --git a/inc/field.class.php b/inc/field.class.php
index 039e812f..44bf2cd8 100644
--- a/inc/field.class.php
+++ b/inc/field.class.php
@@ -1283,6 +1283,18 @@ public static function prepareHtmlFields(
                 $value = is_array($decoded) ? $decoded : [];
             }
 
+            if ($field['multiple'] && is_array($value)) {
+                // Flatten any nested arrays caused by corrupted DB data (double-encoded values)
+                // so that Dropdown::show() always receives a flat list of scalars.
+                $value = array_values(array_filter(
+                    array_merge(...array_map(
+                        static fn($v) => is_array($v) ? array_values($v) : [$v],
+                        $value,
+                    )),
+                    is_scalar(...),
+                ));
+            }
+
             $field['value'] = $value;
         }