Thanks! I can repro your issue but only when the Patreon app is installed on the device. On a device without the Patreon app, I could complete the login process just fine.
Unfortunately, we can’t fix this on our end. In the app, we are using Apple’s aswebauthenticationsession protocol. Once that popup shows up, the OS handles the authentication, and it looks like it redirects to the Patreon app when that app is installed, which then loads Safari. DiscourseHub needs the authentication to complete inside of the initial popup.
Your only option is to uninstall the Patreon app on the device, sign in, and then install it again. (In the DiscourseHub app, you only need to do this authentication process once, when the site is added to the app.)