Function not behaving same when called second time

[jkthappeta]

In my guessing game, when the correct (five-letter) word is entered on the first try, it works fine (which means, it returns the correctly guessed word).

But, when the correct word is entered on any other than the first try, it is returning None instead.

import random


def word_list():
    with open("MyFile.txt", "r") as file:
        allText = file.read()
        return list(map(str, allText.split()))
    
def random_word():
    rnd_word = (random.choice(word_list()))
    return rnd_word

def is_real_word(guess, valid_words_list):
    if guess in valid_words_list:
        #check
        print("Passed word from next guess: ",guess)
        return True        
    else:
        #check
        print("Passed word from next guess: ",guess)
        print("That's not a real word!")
        next_guess(valid_words_list)

def next_guess(word_list):       
    ges_word = input("Please enter a guess: ")
    
    if is_real_word(ges_word, word_list):
        #check
        print("next guess true ges word: ",ges_word)
        return ges_word 
     
def check_guess(s1, s2):
    s = ''
    valid_list = list(s2)
        
    for i in range(len(s1)):
        if s1[i] in valid_list:
            if s1[i] in s2:
                if i == s2.index(s1[i]):
                    s+="x"
                    valid_list.remove(s1[i])                    
                else:
                    s+="o"
                    valid_list.remove(s1[i])                  
        else:
            s+="_"            
    return s

        
        
def play(): 
            
    guessed_word = next_guess(word_list())
    #check
    print("Passed from next guess: ",guessed_word)
    correct_word = random_word()
    outcome = check_guess(guessed_word, correct_word)
   
    print("Random word: ", correct_word)
    print(outcome)
    
play()

Behavior when the correct letter is entered on the first try:
Please enter a guess: kumar
Passed word from next guess: kumar
next guess true ges word: kumar
Passed from next guess: kumar
Random word: taunt
_o_o_

Behavior when the correct word is entered on the second try:
Please enter a guess: aaaaa
Passed word from next guess: aaaaa
That's not a real word!
Please enter a guess: kumar
Passed word from next guess: kumar
next guess true ges word: kumar
Passed from next guess: None