In fact, it is normal for some work to be completed after the buyer takes possession. That's because some things require suitable weather or other conditions that don't coincide with the closing date. That's because builders sod several lots at a time. Therefore, they will wait until several lots are ready. Also, sodding might have to wait for suitable weather conditions.
Many exterior items require suitable weather in order to be properly done.
They include painting, caulking, mortar work, and shingling, to name a few. When you are prepared for the common delays and disruptions that can happen with a new home, you will be better able to deal with them. Assuming you have chosen a reputable builder, your best strategy is communication. Most builders are committed to complete their customers' homes. However, some of them fail to communicate that commitment.