Certified GKH Gardening Expert
Most commonly, this can be attributed to overwatering.
This will be very common when they come from a nursery since they change environments very quickly from there to someone's home. This is very stressful, and they will not use much water during this time of adjusting.
They will need to be allowed to dry out between waterings down to about 3 inches or so before watering again.
You may find that you will need to treat with a fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will help you to care for Poinsettia indoors: https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/poinsettia-care-after-christmas.htm