I believe I’m seeing the same problem. 
My various maps keep getting corrupted after a couple hours of work. What happens is if the map is “corrupted” while I’m still editing it, I won’t know until I press the “PLAY IN EDITOR” button, upon which unreal will crash. If I then load the editor again and try to load that map, it’ll instantly crash and I’ll never be able to load it again. This has been happening on various maps, but if you look at the crash comment it appears to be related to foliage.
Here’s the crash comment :
Fatal error: [File:D:\Build++UE4+Release-4.12+Compile\Sync\Engine\Source\Runtime\Foliage\Private\FoliageInstanceBase.cpp] [Line: 90]
Instance base cache - integrity verification(3): Counter: 22 Size: 17, InvSize: 6 (L:1 S:0 T:0)
As for the foliage in that map, I’ve noticed that if I paint foliage on a static mesh and then duplicate the mesh, the foliage gets duplicated with it. I’m not sure if it’s supposed to do that or not, but one of the last things I did before the map was corrupted was I deleted some foliage from the map. Both from the original mesh and the duplicate mesh, then I added some other foliage and manually moved some of the plants around. I’m assuming that deleting foliage (by shift-painting them) is what’s causing the corruption, but maybe it has to do with there being duplicate foliage when you duplicate the mesh and when you delete some of those, that’s when it might be breaking a foliage list?
Here’s the map and log file and I hope they help :
https://dl.dropboxusercontent.com/u/1615250/UE4_mapLoadCrash.
Thanks!
-seneca