Custom print function has undesired behaviour

Question

I'm making my own x86 OS using the i386-elf cross-compiler and linker and nasm to compile asm files. The OS itself runs with qemu. That being said, I made a custom print function but ran into a problem. Every time I access memory (either through the [] operator or by dereferencing a pointer) and call my print function afterwards, it leaves 8 blank spaces and then prints normally. Print code:

void printv(char *str, ...)
{
    unsigned int tmp_cursor = get_cursor_position();
    cursor_position.x = (unsigned short)(tmp_cursor >> 16);
    cursor_position.y = (unsigned short)tmp_cursor;
    char buffer[12];

    va_list list_ptr;
    va_start(list_ptr, str);
    
    unsigned int i = 0;
    for (char *ptr = str; *ptr != '\0'; ptr++)
    {    
        switch (*ptr)
        {
        case '%':
            cursor_position.y += (cursor_position.x + i) / 80;
            cursor_position.x = (cursor_position.x + i) % 80;            
            update_cursor(cursor_position.x, cursor_position.y);
            i = 0;
            switch (*(ptr + 1))
            {
            case 'c':
                buffer[0] = (char)va_arg(list_ptr, int);
                buffer[1] = '\0';
                printv(buffer);
                ptr++;
                break;
            case 's':
                printv(va_arg(list_ptr, char *));
                ptr++;
                break;
            case 'i':
            case 'd':                
                int_to_str(va_arg(list_ptr, int), buffer, 10);
                printv(buffer);
                ptr++;
                break;         
            default:     
                *(char*)(0xb8000 + (cursor_position.x + i + cursor_position.y * 80) * 2) = *ptr;
                i++;       
                break;
            }  
            break;          
        case '\n':
            i = 0;
            cursor_position.x = 0;
            cursor_position.y++;
            break;
        case '\t':
            cursor_position.y += (cursor_position.x + i) / 80;
            cursor_position.x = (cursor_position.x + i) % 80;                    
            update_cursor(cursor_position.x, cursor_position.y);
            i = 0;
            cursor_position.x += TAB_SPACE - cursor_position.x % TAB_SPACE - 1;        
            break;
        default:
            *(char *)(0xb8000 +(cursor_position.x + i + cursor_position.y * 80) * 2) = *ptr;
            i++;
            break;
        }     
    }

    va_end(list_ptr);
    memset(buffer, '\0', 12);
    cursor_position.y += (cursor_position.x + i) / 80;
    cursor_position.x = (cursor_position.x + i) % 80;  
    update_cursor(cursor_position.x, cursor_position.y);
}

Call example:

    printv("Starting PonchOS!\n");
    char str[12];
    for (int i = 0; i < 11; i++)
    {
        str[i] = 'a' + i;
    }
    str[11] = '\0';
    
    printv("Testtesttesttesttest");

Output:

As you can see, it prints fine before any memory access, but after that, it leaves those white spaces. Any ideas as to why this happens?

Edit:
Implementing @chqrlie 's changes, some issues have been fixed, although spacing problems persist.
Code:

printv("Starting PonchOS!\n");
printv("%c\n", 'C');
printv("%i", 128);
printv("%s", "string");

Output: