Why do leaves turn brown, dry up and fall off?
Your Morning Glory could simply not be getting enough water (perhaps combined with hotter-than-normal temperatures?), or it may be something more serious such as rust or canker. This article may help you identify and treat whatever is ailing your plant:
And these articles may also help you: https://www.gardeningknowhow.com/ornamental/vines/morning-glory/growing-morning-glories.htm