Nothing obvious, post the rest of the code (Specially startupMessage init) or upload the project, never had any issue with this.
Also, normally you don't transmit the null termination:
HAL_UART_Transmit(&huart1, startupMessage, 12, 10000);
Try:
HAL_UART_Transmit(&huart1, "Hello World!\r\n", 14, 10000);