>>It occurs to me that we're using the WebBrowser control and it may not support this parameter, since it's old....
I believe the WebBrowser control default is IE7 which is very old and possibly has bits missing in newer versions of Windows. So if there's inconsistent behaviour between machines or a suspicion that some parameters don't always work the same, you can use a HTTP meta tag that instructs the webbrowser control to emulate a newer version of IE, or even Edge.
<meta http-equiv="X-UA-Compatible" content="IE=11">
Inserting this inside the <HEAD> section may well restore functionality, since IE11 was supported until quite recently. You might even consider IE=Edge that is very unlikely to exhibit differing behaviours in newer OS. Certainly worth a try!
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
