I know this code does not look good and I want to overhaul it anyway, but can somebody please explain to me why this happens:
If I give “aa1f” as input it gives back “Invalid”, as it should,
but if I give “aa1a” as input I get “Valid” as Output.
I’m really confused why this happens and want to understand it before I rewrite it.
Thank you all in advance!

def main():
    plate = input("Plate: ")
    if is_valid(plate):
        print("Valid")
    else:
        print("Invalid")


def is_valid(s):
    #plate must be longer than 2 symbols
    if len(s) < 2:
        return False
    #first 2 symbols must be letters
    for i in range(2):
        if s[i].isalpha() == False:
            return False
    #plate must >=2 and <=6 symbols long
    if 2 <= len(s) <= 6:
        pass
    else:
        return False
    #all symbols must be either numbers or letters
    for i in range(len(s)):
        if s[i].isnumeric() == False and s[i].isalpha() == False:
            return False
    #
    first_num = len(s) - 1
    for i in s:
        if i.isnumeric():
            if i == "0":
                return False
            first_num = s.index(i)
            break
    for character in s:
        if s.index(character) <= first_num:
            pass
        else:
            if character.isalpha():
                return False
    return True


if __name__ == "__main__":
    main()