Runas verb not elevating on specific Windows 10 machines

I have a really odd issue, where some code that we have been running for years has stopped working on certain machines. There is no obvious pattern to the machines (in terms of Win10 build number or patch level).

The code is designed to run an application as the local administrator, and is as follows:

string strPwd = "MySecretPassword";

SecureString securePassword = new SecureString();
foreach (char ch in strPwd)
{
    securePassword.AppendChar(ch);
}

ProcessStartInfo processInfo = new ProcessStartInfo();
processInfo.UserName = "Administrator";
processInfo.Password = securePassword;
processInfo.Verb = "runas";
processInfo.FileName = "cmd.exe";
processInfo.WorkingDirectory = @"c:\windows\system32";
processInfo.UseShellExecute = false;
Process.Start(processInfo);

If I run this on a "working" machine, I see the following (note, the echo test > c:\test.txt is how I am testing whether it can perform a restricted task):

Note the "Administrator" on the CMD window. Also, note that creating a file at the root of c: is allowed.

In comparison, if I run the same on another machine I get the following:

Note that in this case (running exactly the same code) the window is not running elevated. Oddly, if I look at the process in Task Manager, the owner is Administrator.

On both machines:

1. I am using the default Administrator account
2. The password is correct
3. The account is in the Administrators group
4. The account is not disabled

Furthermore, on the "bad" machine, if I right-click and choose RunAs and enter the Administrator credentials - it works perfectly. (Same result as in the first picture above.)

As an additional test I've tried using the DOS runas command and this doesn't work on the "bad" machines either. The result is slightly different though:

Note in this case the "Running as Administrator" in the title bar. (The same test on a "good" machine gives the same result as in the first picture above, i.e.: Administrator: C:\windows\system32\cmd.exe)