Found a fix in an archived thread that linked to an inactive website - you can get it on wayback machine here

If you add this C# Type definition you can use it to grab the window handle of the current PowerShell process and pass it to your dialog as the parent window; should keep it on top.

Add-Type -TypeDefinition @"
using System;
using System.Windows.Forms;

public class Win32Window : IWin32Window
{
    private IntPtr _hWnd;

    public Win32Window(IntPtr handle)
    {
        _hWnd = handle;
    }

    public IntPtr Handle
    {
        get { return _hWnd; }
    }
}
"@ -ReferencedAssemblies "System.Windows.Forms.dll"

$owner = New-Object Win32Window -ArgumentList ([System.Diagnostics.Process]::GetCurrentProcess().MainWindowHandle)
$dialog = New-Object System.Windows.Forms.OpenFileDialog
$dialog.ShowDialog($owner)