I've been trying to determine why Edge is rendering .white-text black. Turns out when compiling with gulp-sass based on node-sass many different selectors extend .white-text, which results in them being joined into single bulky rule. If just one of those selectors has ::placeholder then all other selectors in the same rule are completely dropped by Edge.

When compiling MDB I get this rule that sets color to white:

.white-text, .card.card-cascade .view.view-cascade.gradient-card-header, .dropdown .dropdown-menu .dropdown-item:hover, .dropdown .dropdown-menu .dropdown-item:active,
.dropup .dropdown-menu .dropdown-item:hover,
.dropup .dropdown-menu .dropdown-item:active,
.dropleft .dropdown-menu .dropdown-item:hover,
.dropleft .dropdown-menu .dropdown-item:active,
.dropright .dropdown-menu .dropdown-item:hover,
.dropright .dropdown-menu .dropdown-item:active, .side-nav, .side-nav .search-form input[type=text]::placeholder, .side-nav .search-form .form-control, .toast-message a,
.toast-message label, .toast-close-button, #toast-container > div, .picker__list-item--selected,
.picker__list-item--selected:hover,
.picker--focused .picker__list-item--selected, .picker--time .picker__button--clear:hover, .picker--time .picker__button--clear:focus, .picker--time .picker__button--clear:hover:before, .picker--time .picker__button--clear:focus:before, .picker__date-display, .picker__date-display .clockpicker-display .clockpicker-display-column .clockpicker-span-hours.text-primary, .picker__date-display .clockpicker-display .clockpicker-display-column .clockpicker-span-minutes.text-primary, .picker__date-display .clockpicker-display .clockpicker-display-column #click-am.text-primary, .picker__date-display .clockpicker-display .clockpicker-display-column #click-pm.text-primary, .clockpicker-display .clockpicker-display-column .clockpicker-span-hours.text-primary, .clockpicker-display .clockpicker-display-column .clockpicker-span-minutes.text-primary, .clockpicker-display .clockpicker-display-column #click-am.text-primary, .clockpicker-display .clockpicker-display-column #click-pm.text-primary, .darktheme .picker__box .picker__date-display .clockpicker-display, .darktheme .picker__box .picker__date-display .clockpicker-display .clockpicker-span-am-pm, .darktheme .picker__box .picker__calendar-container .clockpicker-plate .clockpicker-tick, .darktheme .picker__box .picker__footer button, .picker__box .picker__header .picker__date-display, .picker__box .picker__table .picker__day--outfocus, .picker__box .picker__table .picker__day--selected,
.picker__box .picker__table .picker__day--selected:hover,
.picker__box .picker__table .picker--focused {
color: #fff !important;
}

In any other browser it works as it should, but Microsoft Edge won't apply the white color to any of the listed selectors until this one is taken out of it

.side-nav .search-form input[type=text]::placeholder