🇬🇧United Kingdom @chapabu

I managed to work this one out. The QueryString URL processor has a check for whether or not the request is an XMLHttpRequest (I assume for AJAX based Views) where there is a subrequest. In my case, this subrequest doesn't exist because I'm doing something a little different.

The fix for my use-case was to strip the 'X-Requested-With' header out of the request for my specific path in an event listener listening on the KernelEvents::REQUEST event. Possibly not the best solution, but it worked for me without risking breaking anything else I may want to do with Facets.