Why could I sometimes experience swelling in the injected areas (usually when I wake up in the morning)?

All fillers attract water and cause swelling. I’m not familiar with the filler that you are describing, but I understand that it’s more difficult to make short chain fillers, like Vycross from juvederm.
Filler does last longer than one year, particularly in areas that don’t move very much. Swelling happens regularly with normal life activities, for example, if you sleep on your front or side, you will typically experience swelling of the face in the morning because of gravity, pulling the fluid downward. Many of my patients have reported fillers being particularly swollen in the mornings during pregnancy, another time of increased swelling.
Hope this helps.