I've tried to run the devserver on https, If the webpack certificate is used the server starts an https server as expected.
If a certificate has been created ( the webpack one isn't used) the server starts over http instead of https.
I've had a quick look to the source code, and I think the issue is https://github.com/developit/preact-cli/blob/2bd02cb8e5c442c0987d90d6cada3a91516fa790/src/lib/webpack/webpack-client-config.js#L126, ssl is an object and not a boolean.
If I'm not wrong that object is set https://github.com/developit/preact-cli/blob/master/src/commands/watch.js#L47.
Thanks