Strange file write behavior

I'm looking to insert the numbers 7777~7781 at position 6000 while at the same time store the strings contained at position 6001~eof to a temporary data structure. The program runs okay. During the first run, the program stores the strings in a vector, but the position 6000 is never modified. However, during the second run, the position is now modified with the numbers 7777~7781. How can I get through this without having to run the file twice? The file I am trying to modify has a randomly generated integers from (1~9999).

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <vector>
#include <stdio.h>
#include <stdlib.h>

using namespace std;

int main() {
    string filename1 = "file1.txt";
    char buffer[100];
    string str1;

    fstream file1;

    vector<int> nums(5);
    int v = 7776;

    file1.open(filename1.c_str(), ios::in | ios::out); //opens file 1

    if (file1.fail()) {
        cerr << "Failed to open the file: " << filename1 << endl;
        return (-1);
    }

    file1.seekg(30000, file1.beg);

    vector<string> nums_after(9999); // 9999 for testing
    int i = 0;

    // store lines 6001~eof in a temp data structure
    while (!file1.eof()) {
        i++;
        getline(file1, str1);
        nums_after[i] = str1;
        cout << nums_after[i] << endl;
    }
    file1.clear();

    // insert 7777-7781
    file1.seekp(29995, file1.beg);
    for (int u = 0; u <= 4; u++) {
        v++;
        nums[u] = v;
        sprintf(buffer, "%d", nums[u]);
        file1.write(buffer, 4);
        file1.write("\n", 1);
    }
    file1.clear();
    file1.close();
    return 0;
}