Strange behaviour of printf with array when passing pointer to another function - c

Question

To study for the exam we are trying to do some exercise from past exams.
In this exercise we get a header file and we have to create a function that read an input file and print onto the stdout only the parts of strings that do not contain digits.
(We have to pass the pointer of the string red to the main function).
We tried to do it with a an array but when printing the first word is empty or has strange characters. Instead doing a malloc allocation works fine.
What is also strange is that printing before everything an empty string will fix the code.
Therefore we don't understand why using an array of char the first word is not printed correctly, although it is saved in the buffer.
Including a printf before the while loop in the main function will reset the problem.
Using dynamic allocation (malloc) and not static allocation (array) will fix the print.
Iterating over the whole array and set all the memory to 0 does not fix the problem.
Therefore the pointer is correct as with printing an empty string it prints it correctly, but I really cannot understand what cause the issue.
Question are:
How it is possible that printing an empty string the print is correct?
Array is allocated on the stack therefore it is deallocated when the program exit the scope, why is only the first broken and not all the words?
#include "word_reader.h"
#include <ctype.h>
#include <stdlib.h>
#include <string.h>
const char * read_next_word(FILE * f) {
char buffer[WORD_MAX_LEN];
char * word = buffer;
for (int i = 0; i < WORD_MAX_LEN; ++i)
buffer[i] = 0;
//char * buffer = malloc(sizeof(char) * WORD_MAX_LEN);
int found = 0;
int c = 0;
int i = 0;
while (!found && c != EOF) {
while ((c = fgetc(f)) != EOF && isalpha(c)) {
found = 1;
buffer[i] = c;
++i;
}
buffer[i] = '\0';
}
if (found) {
return word;
//return buffer; // when use malloc
}
return 0;
}
int main(int argc, char * argv[]) {
FILE * f = fopen(argv[1], "r");
if(!f) {
perror(argv[1]);
return EXIT_FAILURE;
}
const char * word = 0;
//printf(""); // adding this line fix the problem
while ((word = read_next_word(f))) {
printf("%s\n", word);
}
fclose(f);
return 0;
}
the header file contain only the read_next_word declaration and define WORD_MAX_LEN to 1024. (Also include
the file to read (a simple .txt file)
ciao234 44242 toro
12Tiz23 where333
WEvo23
expected result:
ciao
toro
Tiz
where
WEvo
actual result
�rǫs+)co�0�*�E�L�mзx�<�/��d�c�q
toro
Tiz
where
WEvo
the first line is always some ascii characters or an empty line.