You aren't the first one to notice this. There is a bug report for the exact issue here: [SR-12089] Swift 5.2 snapshot: didSet not called · Issue #54525 · apple/swift · GitHub and another report about it on the forums here: Property wrapper observer not firing
Unfortunately it doesn't look like its been picked up yet and the closer to the 5.2 release we get the less chance it will get fixed. I'm tagging in @beccadax as he was the one who created the internal Apple tracking ticket.