I'm having the same problem. At some point today, all snippets have stopped working for me, persisting through restart. Nothing has been updated recently, and I am on the most recent version of Mojave.
Edit: after some more testing, it seems like they suddenly work in some text fields, but not others. For instance, I have an snippet for my company's download page. It works fine in Chrome's address bar, but not in a text field in Help Scout (which is running inside of Chrome). It also doesn't work in Slack's text field (again, running inside of a Chrome tab).