FIXED by enabling Water Plugin + Editor Preferences: Procedural Foliage!
Just had this issue with migrating maps from a UE4.26.1 project to another UE4.26.1 so it wasn’t the UE4 version.
However, the project where I created my map used the new ocean spline and procedural foliage.
Enabling the water plugins and procedural foliage solved the issue so if you have the same issue check your plugins and project settings in the old and new projects to make sure they are aligned.