I notice in a test conversion of a 1.8.2 terrain with multiple patches that splits became visible at the patch boundaries when the project was opened in the 1.9 update. Removing the terrain (and the height map files that are in the scene / terrain data and not automatically deleted without an editor shutdown), and rebuilding the terrain removes the problem - this may not be a viable fix for someone updating manually sculpted terrain from 1.8, so make a backup.
This issue is still present: Issues with terrain LOD distribution
On the plus side, amazing update and a big thanks to everyone for the hard work!