I have only seen one other person with a similar issue but they did not find a solution. In my project that has been brought into ue5 from 4.26, all of my painted foliage is either flying around in the air, or sliding around to or away from the camera as shown here:
This is really frustrating because to repaint this amount of foliage will take me days!
there are “flush” cache commands you can run that might help try that.
No idea about this issue, but shout out to Towers.
Just up the road from me, I used to work there years back.
Had same issue over and over again. Then I found out FOLIAGE IS ACTUALLY PAINTED ON HIDDEN OBJECTS as well as on visible. Nice “feature” cost me about one day of my life 
So to resolve: delete any hidden mesh/terrain, then it works flawlessly. Also make sure you have unchecked painting on other (also potentially hidden) foliage - this makes foliage on top of another foliage.
EPIC: please can you kindly make sure user knows or even better, is able to turn off paining foliage on hidden objects?
