Seems very much like a patch 8-thing to me because I've never encountered this behavior before.
I first noticed this on Astarion at the Last Light campsite in Act II. Upon entering camp, his tent was seemingly empty, but hovering the cursor over where he should've been still changed the cursor to "talk to" mode and I could interact with him (he was behaving normally and visible during the dialogue sequence). Waiting around while at camp would usually make him pop into the world on his own after a while.
Next I had it happen with Lae'zel at the same campsite (I think) and after a while of hovering the cursor over the position where she should've been, she started to emerge from the ground and rose up slowly and dramatically until she was where she should've been in the first place.
Then it also started to happen with NPCs (I think thus far I've only noticed this with traders). Lady Esther was the first one - fast traveling to her location I was a bit surprised to find her missing, but, again, hovering the mouse over the position where she should've been changed it to "talk to" mode and I could interact and trade with her normally. I'm pretty sure traveling between maps and eventually returning to the Monastery-map brought her back.
Latest example is Bareki near the Undercity waypoint in Act III. He was there initially but returning to that waypoint later on, he was missing. Like with the others, I can still interact/trade with him. This has persisted now for two or three in-game days, meaning he hasn't "come back" yet, even though I regularly travel back and forth between the Lower City and Rivington maps, forcing the game to load the new map.
With him, I checked a bit more thoroughly and it turns out that he's still generating his "correct" cone of vision - but his actual model is standing roughly 10 or 20 meters (or more) below ground. Here' a screengrab I posted on reddit:
https://preview.redd.it/invisible-c...c5407dab9a35d71c037b45d8a332dff76220118aI marked his position with a red square since he's so far below ground that it's easy to miss him, even with character outlines/highlights enabled.
