I've seen it in three Korean series and two movies so far. The latest is from 2023 and the earliest from 2016. The ghosts are mostly good. In three cases ML and the FL ghost develop a romance.

I am wondering about the origin of this trope. A quick internet search produced a western romantic comedy from 2005 called "Just Like Heaven", with ML falling for the ghost of a coma patient .  Are there maybe any earlier (asian) productions using this trope?

Check out these search results for the tag Astral Projection. https://mydramalist.com/search?adv=titles&th=12759&so=top
*Though not all the results feature a show where that troupe is a major part, some it is only a little thing.

It has some dramas and movies with this troupe. I've seen Just Like Heaven. Out of those search results, I think Untouchable Love is the most similar to Just Like Heaven, just a bit more somber.

 NoirRoseHeart:
Check out these search results

Thanks, I'll have a look.

I've also watched Just Like Heaven yesterday. Interestingly, the FL not being dead doesn't feel as much off a plot twist as in the k-dramas.