In STM32F4 series,
Have you noticed that when Read Protection is enabled, with RDP Level 1,
The System Memory Boot mode has no response at least from UART1 and UART3.
(I confirm that TX pin has no response, as checked by oscilloscope. )
I've tried F446 and F410 Nucleo 64 Board on UART1 PA9/PA10 or UART3 PC10/PC11.
However, once the read protection is disabled, RDP back to 0, every thing works again.
(If I send 0x7F through UART, I got 0x79 as expected. )
If I remember correctly: in L4 series, even if Read Protection is enabled, the MCU still responses through UARTx.