Using the same function yields different results?

Going from Framework 4.5 to Net 6.0 for an old project, there is an function that has broken immensely. When using the function for the 1st time, it returns the value 1651. Running the same function for a 2nd time, it returns a 0 value. This has caused an issue where our program cannot open the PKG file properly (it fails reading the 2nd value count.). I am wondering if there's a quick fix to be done?

The File Check (The code using the broken function): Github ArkPackage.cs

private void readNewFileTable(Stream header, bool readHash = false)
{
  uint numFiles = header.ReadUInt32LE();
  var files = new OffsetFile[numFiles];

  for (var i = 0; i < numFiles; i++)
  {
    // Version 3 uses 32-bit file offsets
    long arkFileOffset = header.ReadInt64LE();
    string path = header.ReadLengthPrefixedString(System.Text.Encoding.UTF8);
    var flags = header.ReadInt32LE();
    uint size = header.ReadUInt32LE();
    if (readHash) header.Seek(4, SeekOrigin.Current); // Skips checksum

    var finalSlash = path.LastIndexOf('/');
    var fileDir = path.Substring(0, finalSlash < 0 ? 0 : finalSlash);
    var fileName = path.Substring(finalSlash < 0 ? 0 : (finalSlash + 1));
    var parent = makeOrGetDir(fileDir);
    var file = new OffsetFile(fileName, parent, contentFileMeta, arkFileOffset, size);
    file.ExtendedInfo["id"] = i;
    file.ExtendedInfo["flags"] = flags;
    files[i] = file;
    parent.AddFile(file);
  }

  var numFiles2 = header.ReadUInt32LE();
  if(numFiles != numFiles2)
    throw new Exception("Ark header appears invalid (file count mismatch)");
  for(var i = 0; i < numFiles2; i++)
  {
    files[i].ExtendedInfo["flags2"] = header.ReadInt32LE();
  }

The Broken Function (L154 is a variation that uses 161-171): Github StreamExtensions.cs

public static uint ReadUInt32LE(this Stream s) => unchecked((uint)s.ReadInt32LE());

/// <summary>
/// Read a signed 32-bit little-endian integer from the stream.
/// </summary>
/// <param name="s"></param>
/// <returns></returns>
public static int ReadInt32LE(this Stream s)
{
  int ret;
  byte[] tmp = new byte[4];
  s.Read(tmp, 0, 4);
  ret = tmp[0] & 0x000000FF;
  ret |= (tmp[1] << 8) & 0x0000FF00;
  ret |= (tmp[2] << 16) & 0x00FF0000;
  ret |= (tmp[3] << 24);
  return ret;
}

Before (Framework 4.5):

After (Net 6.0):