EDIT

For anyone else looking for this in the future the change I had to make was in the Python side, using pyserial instead of the version provided by the webiopi.

This is the final code:

import serial

# Configure the serial port (replace with your port and baud rate)
ser = serial.Serial('/dev/ttyACM0', 9600, timeout=1)

@webiopi.macro
def getSerialArduinoData():
    json = ""

    webiopi.sleep(1)

    ser.write(b'd') #send command "d" to request data from Arduino

    data_bytes = ser.read_until(b'\n') # Read until the newline character

    json = data_bytes.decode('utf-8') # Decode the bytes to a string
    json = json.strip() # Remove the delimiter from the end
    return json

EDIT

For anyone else looking for this in the future the change I had to make was in the Python side, using pyserial instead of the version provided by the webiopi.

This is the final code:

import serial

# Configure the serial port (replace with your port and baud rate)
ser = serial.Serial('/dev/ttyACM0', 9600, timeout=1)

@webiopi.macro
def getSerialArduinoData():
    json = ""

    webiopi.sleep(1)

    ser.write(b'd') #send command "d" to request data from Arduino

    data_bytes = ser.read_until(b'\n') # Read until the newline character

    json = data_bytes.decode('utf-8') # Decode the bytes to a string
    json = json.strip() # Remove the delimiter from the end
    return json

Can anyone point me in some directions on why this could be happening?

Thank you in advance.

Why could this be happening?