Hi,
Sorry about these troubles. We found several cases where Editor operations were breaking some assumptions in foliage (some of Undo/Redo operations, level SaveAs, level Duplicate). These should be fixed with CL# 3032336, 3057039 (scheduled for 4.13). Could you integrate these changes? These changes also remove asserts in foliage at loading time, so even if level was already saved with some broken foliage, Editor will still load that level without crashing. Broken foliage will lose a link to a component on which it was painted on (moving that component will not move foliage instances). These can be postfixed in Editor by Snapping foliage instances to ground (select instances + End key). If you can’t integrate those CLs I would recommend to replace all entries of:
UE_LOG(LogInstancedFoliage, Fatal
to
UE_LOG(LogInstancedFoliage, Error
in FoliageInstanceBase.cpp. That at least will allow to load crashing level in the Editor.