I am currently at work, so I will translate the screenshots later.
Thank you very much for testing 8V and 10V and confirming that the UART signal is always 5V. With this, I can discard the issue of voltage and try others ideas.
Considering the "10": it is nothing about the servo itself, but about UART protocol in general. Reading a bit more about the subject: UART send a "0" before each bytes and a "1" after each byte (so we see the "10" between them). This is used to synchronize clocks in the serial communication. you don't need to add them to the string, it is automatically done by the UART chip. However, when we read with the oscilloscope, it is apparent and we need to consider it.