When you are inserting the detail blocks, the insert information is saved to special files of the detail library.

After all, it could be that you did not open the model when inserting blocks and opening the drawing in the background can be quite frustrating because very large models especially require a long computing time for this operation.

Every time you open the model, the model drawing is compared and synchronized with these data. You should never delete these files until the synchronization process is finished and you have saved the model. The modification mechanism cannot function reliably if the program lacks the information telling it which part has been inserted into which drawing.