I got same result as you when using split as I am getting Null result, another option I can suggest to only get the datetime is:

import re
from datetime import datetime

filename = "This_is_new_file_2024_12_06T11_00_49_AM.csv"

match = re.search(r"\d{4}_\d{2}_\d{2}T\d{2}_\d{2}_\d{2}", filename)
datetime_string = match.group(0) if match else None

if datetime_string:
    datetime_object = datetime.strptime(datetime_string, "%Y_%m_%dT%H_%M_%S")
    print(datetime_object)
else:
    print("Datetime not found in the filename")