PL-2303TA TTL UART data corruption
I could not reproduce your issue using my PL2303 USB cable. There must be something wrong with yours.

Maybe you can verify that you are receiving the wrong data also within RealTerm connecting TX to RX instead of just the scope. You should use RealTerm 3.x for better results (the 2.x version I had was not working)

You can see a couple of screenshots with the results I got.