I made a very simple project in Unity/HoloLens that has 2 scanned objects and 1 image as targets.
My simple goal was to attach a basic shape to the origin of each one of the targets.
After playing with the settings I am able to see holograms associated with each of the three targets. For the image the attached sphere is at the origin, everything there works just fine. For the scanned objects I am getting funny behavior. I use extended tracking and set the max number of tracked of objects to 2. This is so I can step back and see where the holograms are in relation to both of the scanned targets. For the scanned targets I used the mars habitat from the examples and I also scanned in an Altoids box myself. I placed the two about next to each but so that only one is in view at a time. I oriented the objects so that the origin of each is in the lower left corner. Now the weird part. The sphere associated with the Altoids box floats above the origin whereas the cube associated with the Mars Habitat floats below the origin and slightly offset. (As an aside the location of the attached sphere and cube are consistent, above and below, whether I use object tracking or not and whether I track 1 or 2.)
Are than any special setting to get the sphere and cube to attach to the origin of their respective targets. They are children of their respective targets with transform position all zeros.