How to interpret scope waveform of UART STM32
UART sends least significant bit first.
The scope diagram matches exactly with the transmission of the 0xA8 or 0b10101000.
It includes a start bit of logic 0, the data bits as 00010101, and stop bit of logic 1 which continues as idle logic 1 between data frames.
Baud rate is measured by the scope, it is 41667 bits per second, so you have some clock or baud rate setting issue.