The project was updated from 4.7 to 4.8 but the map was not. I started the terrain in 4.8.
Before noticing I worked on a far away area of the terrain, just sculpting and terrain painting. I had then a problem with terrain painting, my dirt painting sometimes painted grass instead. So I just saved and restarted the editor which made painting work fine again but then I noticed the floating foliage objects. I can’t tell when that got corrupted.
I tried using the Reapply tool with a slight difference in Min and Max of Z Offset. Those floating object just remain floating with slightly changing the position. So it appears the engine thinks they are on the ground even though they are not. New painted foliage objects behave normally.