Some people have been experiencing an iPhone XS hotspot bug (or maybe just one person). In any case, the issue was solved.
iPhone XS Hotspot Bug
I don’t know if it’s an issue with iOS 12, the iPhone XS, or macOS Mojave. But Christina Warren took to Twitter the other day saying that her iPhone XS kept connecting as a hotspot when she plugged it into her Mac running Mojave.
But Twitter user Chris Ball had a fix: Reset your network settings on the iPhone by doing to Settings > General > Reset. So if you run into this bug, try resetting your network settings.