Well, the game ceilings the XP. Thus, you’re probably earning 0.00001 XP, and that doubles, and still is under 1.
Anyway, as ontheleft has already stated, it’s not a bug. You’ve just exceeded the XP of the zone you’re in.
There’s a misconception that enemies will spawn at you’re level. This is false. And don’t think of a zone as one contiguous zone. Think of each zone as a series of subzones. A rift is a zone, with each floor being a subzone. But outside of rifts, something like The Cow Level might actually be 9 subzones.
Every enemy that spawns in a subzone is snapshotted to the level you are on spawn on entry. Even enemies that haven’t spawned yet, if they spawn in that subzone, will be that level. Upon entry to a new subzone (e.g. a floor in a rift), you’ll have a new set of enemies which will then snapshot to your newest current level.
That is why, outside the rift, you’re getting “normal” XP. But if you keep killing stuff and gaining obscene amounts of XP, which is what happens on the PTR with the XP buff, you’ll soon gain no XP in those zones either. One small difference though is that if you move further in, you’ll cross an invisible boundary to a new subzone.
A demonstration of how this works:
Along with rift guardians, enemies that don’t naturally spawn will be your current level, such as with Cursed Shrines or Cursed Chests.