I have a strange issue which I can only replicate on Microsoft browsers (Edge and IE11 tested).
As mentioned before, the Scroll Behavior specification has only been implemented in Chrome, Firefox and Opera.
Here's a one-liner to detect support for the behavior property in ScrollOptions:
const supportsNativeSmoothScroll = 'scrollBehavior' in document.documentElement.style;
And here's a simple implementation for cross-browser smooth scrolling: https://nicegist.github.io/d210786daa23fd57db59634dd231f341