Array's data is changed if I don't printf it - c

Question

I am writing a C program on Eclipse to communicate from my ARM Cortex M4-F microcontroller in I2C with its master, another MCU.
In my I2C library, I use a static global variable to store all the parameters of the communication (address, lenghts, data buffers). The issue is that a part (an array containing the data to be transmitted, which are 8 bits integers) of this variable gets modified when the interrupt (Start condition followed by the slave's address on the I2C bus) happens, even before executing the code I put the handler. It gets assigned to 8, whatever the initial value.
I tried to put breakpoints basically everywhere, and a watchpoint on the variable, the changes arises seemingly from nowhere, not in the while loop, and before the call to my_I2C_Handler(), so the interrupt is the cause apparently.
I also tried setting the variable as volatile, but that changed nothing.
I noticed one interesting thing: putting a printf of the array's data during my_I2C_Init() or my_SlaveAsync(), like so:
printf("%d\n", req.tx_data[0]);
corrects this problem, but why? I want to remove all prints after debugging.
#include <stdint.h>
#include "my_i2c.h"
void I2C1_IRQHandler(void)
{
printf("\nI2C Interrupt\n");
my_I2C_Handler(MXC_I2C1); // MXC_I2C1 is a macro for the registry used
}
int main(void)
{
int error = 0;
printf("\nStarting I2C debugging\n");
// Setup the I2C
my_I2C_Shutdown(MXC_I2C1);
my_I2C_Init(MXC_I2C1);
NVIC_EnableIRQ(I2C1_IRQn); // Enable interrupt
my_I2C_SlaveAsync(MXC_I2C1); // Prepare to receive communication
while (1)
{
LED_On(0);
LED_Off(0);
}
printf("\nDone testing\n");
return 0;
}
The structure of the request containing the parameters of the communication is like this:
typedef struct i2c_req i2c_req_t;
struct i2c_req {
uint8_t addr; // I2C 7-bit Address
unsigned tx_len; // Length of tx data
unsigned rx_len; // Length of rx
unsigned tx_num; // Number of tx bytes sent
unsigned rx_num; // Number of rx bytes sent
uint8_t *tx_data; // Data for mater write/slave read
uint8_t *rx_data; // Data for master read/slave write
};
Is declared like so in the beginning of the file:
static i2c_req_t req;
and assigned this way in my_I2C_Init():
uint8_t rx[1] = {0};
uint8_t tx[1] = {12};
req.addr = 0xAA;
req.tx_data = tx;
req.tx_len = 1;
req.rx_data = rx;
req.rx_len = 1;
req.tx_num = 0;
req.rx_num = 0;
Many thanks for your help