Changing the default value of a struct that is included as an array in another struct can prevent any Blueprints that use a DataTable made from the second struct from compiling. It appears to be a permanent issue that requires both involved structs, the DataTable, and the Blueprint to be deleted and re-created.
The compile fails with an error similar to "Error Invalid property 'CallFunc_GetDataTableRowFromName_OutRow' structure 'SecondStruct' error: Struct 'UserDefinedStruct /Game/SecondStruct.SecondStruct' Property 'TheStruct_6_94CBAF3C41107BBF392E338304F95177' Error ( Struct 'UserDefinedStruct /Engine/Transient.STRUCT_REINST_FirstStruct_3' is not compiled )". In addition, the variable names in the datatable have changed to now include apparent random hashes in the names.
The Blueprint compiles successfully.
Head over to the existing Questions & Answers thread and let us know what's up.