"unlabelled" door in the instance segmentation map

[MhdMohammadi]

I was using semantic segmentation and instance segmentation maps, and I realized object door_lvncak_0 in Beechwood_0_garden is marked as unlabelled in instance segmentation map, however, it is correctly detected as door category in semantic segmentation map. I wonder why this issue exists, and how we can access the id of this door when it's observed.