When using the Leather Sewing Kit to patch leather stuff that needs it, a message shows up saying "Leather Sewing Kit cannot be used to mend *that leather object* ". It recognized as a normal Sewing Kit. I tried with different items, always shows up same message. And I don't even have a normal Sewing Kit in game, so it's impossibile it was my mistake.

Update: I used normal Sewing Kit to mend some leather items. {F31969}