STM32 usart interface sends data but "128" is added to each sent byte
Possibly it might be this: -

The above implies that the transmission has a word length of 7 bits plus a parity bit. But, you have parity disabled (none) and so it might only be transmitting 7 bits followed by the stop bit (always 1). That could give your PC the impression that the stop bit is a data bit and that it is high. Have you tried setting your PC to receive 7 data bits and no parity bit?