Spoofed user agent not sent to proxy server ?

[eblis]

We have a company proxy that only allows Google Chrome and Microsoft Edge browsers.
Most traffic goes through these proxies so my Windows machine is configured to use a proxy script when connected to company VPN.

I want to use Vivaldi browser and change the user agent to chrome to trick the proxy into allowing me to browse via Vivaldi.
This used to work up until a few weeks ago where it stopped working and I don't know why.

For example, if I try to connect to github to view one of these issues while connected to VPN I get a proxy error like so:

The user agent switcher popup says that it's in default mode

even though I've configured it to use a custom Chrome UA:

However, if I load the same page while disconnected from the VPN and then connect to VPN the page will now work for a time and user agent switcher will report that the custom UA is used.