Bug
Recorded on SlateJS.org
Firefox 62.0 :

Safari 12.0.2 :

Recorded in JSFiddle:
Firefox 62.0:

Safari 12.0.2:

In all examples, focus changes are required to enable spellcheck. In both, I'm doing a combination of clicking inside and clicking outside of the Slate editor component and clicking on the words themselves. The issue behaves a bit differently in each browser, but it's a very similar issue that requires changes in focus to get the red underline to appear.
Something more like how Chrome works, ignoring the flashing decorations of course.
Example:
Chrome 71.0.3

Any help is appreciated! :) Thank you so much!
We're experiencing the same issue and having to look into this, don't suppose anyone would have any pointers or suggestions?
I believe that this may be fixed by https://github.com/ianstormtaylor/slate/pull/3093, which has changed a lot of the logic in Slate and slate-react especially. I'm going to close this out, but as always, feel free to open a new issue if it persists for you. Thanks for understanding.
I am still experiencing this issue in Firefox 72.0.2.
Most helpful comment
I am still experiencing this issue in Firefox 72.0.2.