Here's the response I received from Enpass:
Please note that Enpass is a platform-specific purchase. Enpass has to be bought separately for each platform. However, if you have already made a purchase on iOS, we'll request you to wait and not purchase Enpass for Windows for a couple of days as Enpass is moving to a subscription-based model. This will allow the existing users to restore their purchase on any platform. For more details, please read the blog here.
I was able to upgrade Enpass on my windows device shortly afterwards. Although the message mentions a transition to a subscription model, I haven't been charged anything additional - yet!