Fix LCD button / newbutton issue (#13157)

Marlin/ultralcd.cpp

@@ -5606,15 +5606,14 @@ void lcd_reset_alert_level() { lcd_status_message_level = 0; }
 
         #endif // LCD_HAS_DIRECTIONAL_BUTTONS
 
-        buttons = newbutton;
         #if ENABLED(LCD_HAS_SLOW_BUTTONS)
-          buttons |= slow_buttons;
+          newbutton |= slow_buttons;
         #endif
+        buttons = newbutton;
 
         #if ENABLED(ADC_KEYPAD)
 
           uint8_t newbutton_reprapworld_keypad = 0;
-          buttons = 0;
           if (buttons_reprapworld_keypad == 0) {
             newbutton_reprapworld_keypad = get_ADC_keyValue();
             if (WITHIN(newbutton_reprapworld_keypad, 1, 8))