CS50 2019 Vigenere - second loop

First I got my code to do the following:

key: ab Plaintext: Test me Ciphertext: Tfsu ne

I am ofcourse trying to get my key to skip space, (Tfsu mf), so I tried adding an extra counter (m-counter) for my key, to make sure that it doesn't increment after every iteration over the plaintext. The result, however, is that now I get:

key: ab Plaintext: Test me Ciphertext: Uftu nf

So now it acts like my key is bb.

Could somebody explain the 'logic' why this second loop causes this change in output, instead of only incrementing when a character will be ciphered (i.e. is alphabetic)?

            for (int m = 0; m<l; m++)
                {
                   for(int e = 0; e<z; e++)
                       {
                          if (islower(plaintext[e]))
                           {                                         
                           ciphertext [e] = (plaintext [e] + shift `(argv[1][m%l]) - 'a') %26 + 'a';`
                            }
                           if (isupper(plaintext[e]))
                            {    

                            ciphertext [e] = (plaintext [e] + shift (argv[1][m%l]) - 'A') %26 + 'A';      
                            }  
                       }
                 }

             for (int q = 0; q<z;q++)
                  {          
                     if (!isalpha(plaintext[q]) )
                          {    
                           ciphertext [q] = (plaintext [q]);
                           } 
                  } 


printf ("ciphertext: ");
for (int i = 0; i<z; i++)   
    { 
    printf ("%c", ciphertext [i]);
    }
 printf("\n");

This loop for (int m = 0; m<l; m++) tells the program to cipher the entire plain text with key[0], then cipher the entire plaintext with key[1], etc through l. Which exactly describes the result you see (ie looks like it is ciphering with "bb", because it actually is!)

Generally in this assigment, one should not use a loop to control the key index. It should be incremented when used (and of course "wrap around" at the lenght).