Repro on latest, tested today. A user reported a problem with Event cards not displaying, inexplicably. The topic saved normally, the Event markup was correct in the raw post, but no Event card appeared in the topic or elsewhere. There was no composer warning, save error, browser-console error, or relevant /logs entry.
I’d be in favour of b) a clear warning or validation error
While we’re here, the exact same behaviour (silent failure to display the card) also happens if for any other reason the Event markup can’t be parsed properly. In the case I was investigating a few months back, a user had diligently prepared their Event markup in Word then copy-pasted it into Discourse, resulting in automatic “Smart Quotes” around data= items, with the ensuing hilarity you’d expect.
So yes, a visible error, even if it’s just in the console, would be reeeeaally nice.