Thank you @mikel!
I still need the other CSS property transitions, so I pulled just the opacity transition and it works great!
.slds-notification {
position: relative;
width: 20rem;
border: 1px solid #ecebea;
border-radius: .25rem;
box-shadow: 0 2px 3px 0 rgba(0,0,0,.2);
transition-duration: .4s;
transition-timing-function: ease-in-out;
/* transition-property: margin,max-height,opacity,top; */
transition-property: margin,max-height,top;
}
Greatly appreciated