Hey @RoryG42, welcome to the forums! I’ve seen this issue a couple of times, and there are a ton of possible problems. Since it was functioning fine for you in UE4, I’m going to assume it’s either the new shadow mapping, Ray traced shadows, or LOD’s.
For the first 2 possibilities this post links to a good video that runs through a bunch of solutions. Foliage issue - #2 by ClockworkOcean
Lastly, if you can play with the static mesh’s LODs and see if any of it’s possible LOD’s stop this from occurring.
Hope this helps, let me know if you find anything out or if any of these options work!