This is STILL an open ongoing bug within the unreal engine and EPIC doesn’t know how to duplicate it. I can duplicate it by making a new C++ project in 4.21.1, setting a custom GameState and gamemode, making a new struct and datatable based on the struct , then writing blueprint code in the GameState that references the datatable. Then you cook all, then add a new row to the struct, then cook all again and BOOM . Undefined struct errors galore until you recompile blueprints or rename structs.