I am using sync, yes. It’s strange, in my inbox your spoiler format appears correctly as the colons, but in the thread it appears as greater exclamation mark. I think sync is not only using a different tag to make it work, but live changing other users’ formatting 🤔.
I checked my post in browser and it was changing the less than sign to the HTML character code do that may also be the issue.
Or the person telling the story lies some of the time in which case it’s not a riddle and you’re just talking to a normal person