Strange array assignment behavior in C

I have in my C code an array of elements inside an heap structure defined as

typedef struct Elem {
    void    *key;
    void    *value;
} Elem;

typedef struct Heap {
    unsigned int    size;
    unsigned int    capacity;
    Elem            **array;
    compareFunction compare;
} Heap;

For the element insertion function, I use this pseudo-code for add a custom element inside the heap array

Insert (H, x)
HeapSize(H) := HeapSize(H)+ 1
i := HeapSize(H)
H[i] = x
while i > 1 and H[Parent(i)] < H[i] do
    exchange(H[i], H[Parent(i)])
    i := Parent(i) 

But during the assignment, especially after adding 4 elements, on the 5th call of this function a strange behavior happens in this line H[i] = x

for (k = 0; k < hpSize(hp) - 1; k++)
    printf("%dn", *((int *)hpGetEl(hp, k)->key));
if (hpSize(hp) > 1)
    printf("%dn", *((int *)(hp->array[0])->key));
printf("i: %dn", i);

hp->array[i] = hpCreateElem(key, value);

printf("%dn", *((int *)(hp->array[0])->key));
for (k = 0; k < hpSize(hp); k++)
    printf("%dn", *((int *)hpGetEl(hp, k)->key));

The output of the debugging printf is

1
4
2
3
---
1
i: 4
4198800
---
4198800
4
2
3
7

The assignment change the value of the key inside the first element of the array and insert the new element on the 5th position. How is this possibile?

The hpCreateElem function is defined as

Elem *hpCreateElem(void *key, void *value)
{
    Elem *el;

    el = (struct Elem *) malloc(sizeof(struct Elem));
    if (el == NULL) return NULL;

    el->key = key;
    el->value = value;

    return el;
}