Please add the below CSS codes on Appearance > Customize > Layout/Design Options > Custom CSS
h1, h1 a, h2, h2 a, h3, h3 a, h4, h4 a, h5, h5 a, h6, h6 a {
text-transform: none !important;
}
Hope, this will solve your problem.
Let us know about the status after using this code.
Regards
