Sending constant int from Arduino to PC causes error

Question

I am using Arduino Uno Rev 3, and according to this link we can achieve 2000000 baud rate.

This problem can be easily repeated by the following code:

void setup() {
  Serial.begin(2000000);
}

void loop() {
  int val = 234;
  Serial.println(val);
}

I am using the serial-USB cable. To read the data, I used Python pyserial (< 20 lines of code including empty lines and import lines):

import serial
import matplotlib.pyplot as plt

LEN = 10000
data = []
ser = serial.Serial('COM3', baudrate=2000000)
for i in range(LEN):
    try:
        data.append(int(ser.readline()))
    except:
        pass
ser.close()

# %% Plot data
plt.figure(figsize=(6, 4))
plt.plot(data)
plt.savefig('data.png')

Here is what I got:

Edit below

Chris mentioned a good point that it might be the high baud rate of 2,000,000 causing the instability. I did think about that when I first ran into this, but then I did a little trick: adding a sinusoid into the constant. If I do that, the weird spiking goes away! I think it indicate that this is not a high baud rate issue... Is it?

Code:

void setup() {
  Serial.begin(2000000);
}

void loop() {
  int val = 234;
  val += int(512 * (sin(float(millis()) * 0.01) + 1)) * 0.1;
  Serial.println(val);
}

Output: