Why did a watch face disappear from my watch?
Your watch can only hold a handful of faces at once. Here's what happens when a new one arrives.
About 3 minutes
Wear OS watches only have room to keep a small, fixed number of watch faces installed at once. When you install a new one and you're already at that limit, the oldest one is quietly removed to make room. This is how every watch face push app works, not something specific to Face Hub, and not a bug.
The face itself isn't gone, though. It's still yours if you bought it, and it's still listed in Recently worn even after it's been bumped off your watch. Face Hub keeps that history separately, specifically so a face you liked doesn't just vanish from view the moment your watch runs out of room.
Reinstalling it is one tap from Recently Worn or Owned, the same as installing anything else. It just costs nothing the second time, since you already own it.
It can, the first time: a watch has no obvious "storage full" warning of its own. Once you know it's coming, checking Recently Worn is the fix, rather than something being wrong.