I am trying to write SQL part for CSV in C. Why will the code below distort result without duplicating TCHAR * arguments?

Question

The code I write is in C on Visual studio 2022.. I don't understand what part of my code is wrong, but it should be about string manipulation. The SplitStr splits string by another string (strtok does it by character).

#define SplitCount 20
TCHAR** SplitStr(TCHAR* pstr, TCHAR *psplitby)
{
    
if (pstr == NULL || psplitby == NULL)
        return NULL;

    TCHAR** res = malloc(SplitCount * sizeof(TCHAR*));
    

    int i = 0, j = 0, c = 0;
    TCHAR* splitby = _tcsdup(psplitby); // I need to duplicate here
    int len = _tcslen(splitby);
    TCHAR* str = _tcsdup(pstr);  // and here
    int strlen1 = _tcslen(str);

    while (i < MAXSTR - strlen1) {
        
        if (i + len < strlen1 && _tcsncmp(splitby, &str[i], len) == 0)
        {
            res[c] = malloc(MAXSTR * sizeof(TCHAR*));
            if (res[c] == NULL)
                break;
            _tcsncpy_s(res[c], MAXSTR, &str[j], i-j);
            i += len;
            j = i;
            c++;
            i--;
        }
        i++;        
    }

    res[c] = malloc(MAXSTR * sizeof(TCHAR*));
    if (res[c] != NULL)
    {
        _tcsncpy_s(res[c], MAXSTR, &str[j], strlen1 - j);
        c++;
    }

    res[c] = _tcsdup(_T("END"));

    return res;
}
TCHAR* StrToLower(TCHAR* Str)
{
    int i = 0;
    TCHAR * tstr = malloc(MAXSTR * sizeof(TCHAR));
    ZeroMemory(tstr, MAXSTR * sizeof(TCHAR));

    if (tstr != NULL)
    {
        _tcsset_s(tstr, MAXSTR, _T('\0'));
        for (; i < MAXSTR; i++)
        {           
            tstr[i] = _totlower(Str[i]);
        }
    }

    return tstr;
}
int SelectFromTable(TCHAR* Table, FIELDS fields[], TCHAR* Where);
int execute_sentence(TCHAR* psentence)
{
    
    TCHAR * sentence = StrToLower(psentence); 
    
    StrTrim(sentence, _T(" "));
    
    TCHAR* dlimiter = _T("?");

    if (_tcsncmp(_T("select"), sentence, _tcslen(_T("select"))) == 0)
    {
        
        TCHAR ** split = SplitStr(sentence, _T("where"));
        TCHAR* select = split[0];

        StrTrim(select, _T(" "));
        TCHAR* where = split[1];
        StrTrim(where, _T(" "));
        //_putts(where); // here value is OK
        TCHAR ** split2 = SplitStr(select, _T("from"));
        
        TCHAR * fields = split2[0] + _tcslen(_T("select"));
        
    
        TCHAR * table = split2[1];
        StrTrim(table, _T(" "));
        _putts(where); // here value is distorted now if I don't use _tcsdup above
        TCHAR ** split3 = SplitStr(fields, dlimiter);

        TCHAR* thefield = split3[0];
        StrTrim(thefield, _T(" "));
        
        FIELDS fields_arr[Field_Count];
        int c_field_pos = 0, i = 1;
        
        while (_tcscmp(thefield, _T("END")) != 0)
        {
            StrTrim(thefield, _T(" "));
            
            c_field_pos = 0;

            if (thefield != NULL)
            {


                for (int i = 0; i < Field_Count; i++)
                {
                    if (_tcsncmp(thefield, table_columns[i], _tcslen(table_columns[i])) == 0) {
                        fields_arr[c_field_pos] = (FIELDS)i;
                        c_field_pos++;
                        break;
                    }
                }

            }
            thefield = split3[i];
            i++;

        }
        fields_arr[c_field_pos] = F_END;            

        SelectFromTable(table, fields_arr, where);
    }
    return 0;
}

I never expected the need to duplicate, I even used a pointer and kept it intact, still problem exists..