Nevermind, I seem to be able to make the problem go away by using “cover” instead of “contain” on background-size.

.avia-msie-11 .avia-fullscreen-slider .avia-slideshow > ul > li{
background-size: cover !important;
}
.avia-msie-11 .avia-fullscreen-slider .avia-slideshow-inner{
transform: none !important;
}