Certified GKH Gardening Expert
This is a common issue with potassium and phosphorus deficiencies. Are there any other symptoms, or just this? If there are other symptoms, then it may be a fungal/bacterial issue in the soil. If the leaves appear healthy, then a dose of fertilizer made for mandevillas will correct this.
This article will help to learn when and how to feed these: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/fertilizing-mandevilla-plants.htm