Fixes issue #1303

src/Entities/Entity.php

@@ -57,7 +57,7 @@ public function __construct(array $data, string $bot_username = '')
      *
      * @return array
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return $this->getRawData();
     }
@@ -161,6 +161,7 @@ public function __call($method, $args)
             // Limit setters to specific classes.
             if ($this instanceof InlineEntity || $this instanceof InputMedia || $this instanceof Keyboard || $this instanceof KeyboardButton) {
                 $this->$property_name = $args[0];
+                $this->raw_data[$property_name] = $args[0];
 
                 return $this;
             }

tests/Unit/Entities/KeyboardTest.php

@@ -186,4 +186,23 @@ public function testKeyboardAddRows(): void
         $keyboard = $keyboard_obj->getProperty('keyboard');
         self::assertSame('Button Text 4', $keyboard[2][0]->getText());
     }
+
+    public function testSetterMethods(): void
+    {
+        $keyboard = (new Keyboard(
+            [
+                ['text' => 'One']
+            ]
+        ))->setResizeKeyboard(true);
+
+        $array = json_decode($keyboard->toJson(), true);
+
+        $this->assertIsArray($array);
+
+        $this->assertArrayHasKey('keyboard', $array);
+        $this->assertArrayHasKey('resize_keyboard', $array);
+
+        $this->assertIsArray($array['keyboard']);
+        $this->assertEquals(true, $array['resize_keyboard']);
+    }
 }