I’ve had the same problem.
Invisible foliage but still existing collisions.
The problem is culling.
Check if you have a “Cull Distance Volume” in the level.
If it is on, turn it off.
The foliage should reappear completely.
If something still does not appear, right click on the foliage → Replace With (replace it with a Static Mesh Foliage Any), then again, right click → Replace With (replace the newly replaced foliage with the original one).
Hope I can help somehow