Run some simple numbers.
Your plots indicate ~11% of the period is sample rate, which maps to a sample rate of 450Hz, and that suggests ~9600bd, if sending 16 bits with no framing.
so you are within 2:1 of what I'd call expected.

You did not give full info on how the 16 bits is sent. (parity bit for Upper/Lower or ? )

Check there are no gaps between characters, and try higher Baud rates.
Your 50Hz example shows what would be expected for a 9:1 sample ratio, if that is a problem, then scale as needed. PCs and 8051's will easily manage 6x your claimed Baud speed,