Subsites + Fluent = incorrect URL segment on edit

Version: Silverstripe CWP 2.1.1 (GitHub - silverstripe/cwp-installer: CWP project template)

When using Fluent in tandem with Subsites, the URLSegment shown on any subsite edit page uses the wrong base domain (making it difficult for the editor to open the page in another tab).

e.g. here’s the page without Fluent installed, showing the correct URL:

…but if I add Fluent to my build, we get this…

Obviously, Fluent is having to do some altering of that URL to add the appropriate language code (in this case /en/), but that’s conflicting with whatever Subsites is also doing to get the correct domain showing.

Any suggestions?