What are the pitfalls of memmove on gcc STM32?

Question

C++ / C#

Greetings!
You need to shift the array left and right (not cyclically, protruding elements are removed).
To do this, I made the following code:

int N = MIN( (len-index), (QUEUE_SIZE-index2) ); 
 memmove(buf+index2, buf+index,  N * sizeof(int) );

Where

• buf - static array (for example, int is specified, in real life void *),
  
• QUEUE_SIZE - its length
  
• len - the index of the last real element
  
• index - from where
  
• index2 - where
  
• N - calculates how many elements to move
  

This is how it looks like:
QUEUE_SIZE = 12
len = 10
m[12]= 1_2_3_4_5_6_7_8_ 9_10_0_0
______|_______|_________________
__________index___index2_____________
index = 2 - where
index2 = 5 - where
The result should be like this:
m[12]= 1 2 0 0 0 3 4 5 6 7 8 9
This is how it turns out, but on stm32 sometimes NULL appears in buf[0], although it should not be there, and all other elements match in the right places. The length len is the same. What are the pitfalls of memmove() or where could I mess up?
Here is an example code in its entirety for experimentation: cpp.sh/9e436
PS The stack and heap sizes do not overlap each other, there is enough memory.