Isolation in BOM

Hello! I'm trying to do a drawing of the harness assembly, that requires heat shrink tubing in some parts of it. I made heat shrink tubing using the "Add Isolation" and added it to some parts of the route. I also created custom isolation library for this purpose and chose it when creating isolation. The problem is that it appears in BOM, but the lengths of different pieces of the same library item do not sum up (it shows a length of one random piece of isolation). I've tried choosing different properties when editing BOM table, but it didn't help. What I am doing wrong? Is there a way to show the sum of different pieces with the same characteristics?