If I'm in the Lost Grove, leave, then come back, all of the Elites and Named disappear, despite being there before exiting. This has happened the previous two times I've exited the Lost Grove.

Both times I exited through the "exit zone" button, not through the barracks. All spawns have disappeared, no matter where I exited from (at least through the deeper caverns - I haven't defeated the Crystalline Behemoth yet so IDK if it's happening past there).

Yep. This is intentional. When you exit via the "Exit Zone" button, it's considered a 'hard' exit and despawns everything - there is a warning on the popup before confirming.

This is to deter people abusing the exit to teleport around the map.

If you exit via the portal at the entrance or via the Barracks, it will not incur this penalty.

Oh, that makes sense. I normally use the portals back to the barracks, so that would explain why I haven't encountered it before lmao. Thank you!