String class summation disappears the right operand
Your "disappearing string" has the following cause: Out-of-Memory.

You are constructing lots of String objects in your code and are concatinating to one. That uses memory on the heap for each String (and realloc() when the String memory must be expanded).

Consider the following test sketch which includes your code but also adds a memory leak by 150 bytes in each iteration:



#include &lt;Arduino.h&gt;
#include &lt;ArduinoSTL.h&gt;
using namespace std;

extern unsigned int __heap_start;
extern void *__brkval;

/*
 * The free list structure as maintained by the
 * avr-libc memory allocation routines.
 */
struct __freelist {
    size_t sz;
    struct __freelist *nx;
};

/* The head of the free list structure */
extern struct __freelist *__flp;

/* Calculates the size of the free list */
int freeListSize() {
    struct __freelist* current;
    int total = 0;
    for (current = __flp; current; current = current-&gt;nx) {
        total += 2; /* Add two bytes for the memory block's header  */
        total += (int) current-&gt;sz;
    }
    return total;
}

int freeMemory() {
    int free_memory;
    if ((int) __brkval == 0) {
        free_memory = ((int) &amp;free_memory) - ((int) &amp;__heap_start);
    } else {
        free_memory = ((int) &amp;free_memory) - ((int) __brkval);
        free_memory += freeListSize();
    }
    return free_memory;
}

String jsonificar(vector&lt;vector&lt;String&gt;&gt; sujeito) {
    String json_txt = String(""), chave, valor;
    String chave_str, valor_str;

    json_txt += String("{");
    for (unsigned int i = 0; i &lt; sujeito.size(); i++) {
        if (sujeito[i].size() != 2) {
            Serial.println("Cada conteudo deve conter 2 textos!");
        } else {
            chave = sujeito[i][0];
            valor = sujeito[i][1];
            Serial.println(chave);
            Serial.println(valor);
            Serial.println(json_txt);
            json_txt += "\\\"";
            Serial.println(json_txt);
            json_txt += chave;
            Serial.println(json_txt);
            json_txt += "\\\"";
            Serial.println(json_txt);
            json_txt += ":";
            Serial.println(json_txt);
            json_txt += "\\\"";
            Serial.println(json_txt);
            json_txt += valor;
            Serial.println(json_txt);
            json_txt += "\\\"";
            Serial.println(json_txt);

            if (i != sujeito.size() - 1) {
                json_txt += String(",");
            }
        }
    }

    json_txt += "}";
    Serial.println(json_txt);

    return json_txt;
}

void setup(void) {
    Serial.begin(115200);
}

String myStr = "";

void loop(void) {

    Serial.print(F("Free Memory:"));
    Serial.println(freeMemory());

    //force a memory leak by allocating appending to a global String
    myStr +=
            "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
            "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";

    vector&lt;vector&lt;String&gt;&gt; v =
            { { "Chave1", "Valor1" }, { "Chave2", "Valor2" }, };

    jsonificar(v);
    delay(1000);
}


From the output we can see the exact point where we have enough memory and the result is correct, and where we don't have enough memory and the string just "disappears" because there was not enough memory to allocate it:

Free Memory:911
Chave1
Valor1
{
{\"
{\"Chave1
{\"Chave1\"
{\"Chave1\":
{\"Chave1\":\"
{\"Chave1\":\"Valor1
{\"Chave1\":\"Valor1\"
Chave2
Valor2
{\"Chave1\":\"Valor1\",
{\"Chave1\":\"Valor1\",\"
{\"Chave1\":\"Valor1\",\"Chave2
{\"Chave1\":\"Valor1\",\"Chave2\"
{\"Chave1\":\"Valor1\",\"Chave2\":
{\"Chave1\":\"Valor1\",\"Chave2\":\"
{\"Chave1\":\"Valor1\",\"Chave2\":\"Valor2
{\"Chave1\":\"Valor1\",\"Chave2\":\"Valor2\"
{\"Chave1\":\"Valor1\",\"Chave2\":\"Valor2\"}
Free Memory:761
Chave1
Valor1
{
{\"
{\"Chave1
{\"Chave1\"
{\"Chave1\":
{\"Chave1\":\"
{\"Chave1\":\"Valor1
{\"Chave1\":\"Valor1\"
Chave2
Valor2
{\"Chave1\":\"Valor1\",
{\"Chave1\":\"Valor1\",\"
{\"Chave1\":\"Valor1\",\"Chave2
{\"Chave1\":\"Valor1\",\"Chave2\"
{\"Chave1\":\"Valor1\",\"Chave2\":
{\"Chave1\":\"Valor1\",\"Chave2\":\"
{\"Chave1\":\"Valor1\",\"Chave2\":\"
{\"Chave1\":\"Valor1\",\"Chave2\":\"
{\"Chave1\":\"Valor1\",\"Chave2\":\"