It is not unusual for an Amelanchier canadensis ‘Glenn Form’ Rainbow Pillar, commonly known as a Serviceberry, to loose some of its older (interior) leaves, especially after being newly transplanted, if otherwise the leaves do not show any signs of disease. It takes some time for a newly planted tree or shrub to establish its root system in the new environment. During this time it is especially sensitive to too little, but also to too much water. It is important that you keep the soil evenly moist throughout the first two growing seasons. Water regularly when needed, this is when the soil near the stem is dry down to the first inch.
Also, find information about potential diseases in this very similar question posted on our website.
I hope you will be able to enjoy your Amelanchier for a very long time.