Astra support provided some custom CSS to use as an interim solution. The mobile menu is now expanding fully after adding the provided CSS. Here is the code if anyone else is having the same mobile menu issue:
/* To correct Astra Pro 4.0.1 upgrade issue - mobile menu not expanding fully */
#ast-mobile-header .content-align-flex-start {
max-height: fit-content;
overflow: auto;
}