failing to print not matched lines from second file

[menator01]

Using the example above

with open('f1.txt', 'r') as file:
    lines = set([line.strip().split(',')[3] for line in file])

with open('f2.txt', 'r') as file:
    for line in file:
        line = line.strip()
        if line.split(',')[3] not in lines:
            print(f'{line} NO MATCH')

Output:
03/28/2021,P,16,LINE1 NO MATCH
03/28/2021,P,9,LINE3 NO MATCH
03/28/2021,P,8,LINE5 NO MATCH
03/28/2021,S,95,LINE6 NO MATCH
03/28/2021,S,1,LINE7 NO MATCH
03/28/2021,P,46,LINE8 NO MATCH