Inconsistent stack sizes in linker map

When the option '--info stack' is added to the Linker, the generated linker map file contains stack usage for each function.
One function written in C is not calculated correctly:
The function repeatedly calls another function that returns a struct. For each call the stack usage in the map file increases when compiled with compiler version 6 (tested with 6.12, 6.15 and 6.20.1).
Debugging through the function shows that R13(SP) is not increased after each call.
This behavior is not if the compiler version 5 is used, or the file is in C++ (filename ending changed .cpp)
Is there an error in the Linker? We need this value to check automatically if the stack size declared too small.

Examplecode:

#define NUM 300
struct a { union { float f; unsigned short us; } f2u; };
struct b { float sF; int i; };

static struct a arr[NUM];

static struct b foo(int x) {
struct b ret;
ret.sF = 1.F;
ret.i = x;
return ret;
}

void my_init(void) {
int i = 0;
arr[i].f2u.f = foo(1).sF;
i++;
arr[i].f2u.f = foo(12).sF;
i++;
arr[i].f2u.f = foo(71).sF;
i++;
arr[i].f2u.f = foo(42).sF;
i++;
arr[i].f2u.f = foo(13).sF;
i++;
arr[i].f2u.f = foo(1).sF;
i++;
arr[i].f2u.f = foo(12).sF;
i++;
arr[i].f2u.f = foo(2).sF;
i++;
arr[i].f2u.f = foo(2).sF;
i++;
}

int main() {
my_init();
}