Update - Edge has a bug where spellchecker will underline words if misspelled but provide no suggestions. This is a bug with Edge that happened recently (6 days ago). 

Zendesk relies on the browser spellcheck so it does impact Zendesk in a way but not a Zendesk bug: Spell check (Microsoft Editor) has stopped working today (Edge updated this morning) : r/MicrosoftEdge

Solution - Go to  browser settings and enable just the basic, it does fix problem: