Just to note, Unlisted topics aren’t the best for creating a draft topic. If you create one in a permission-controlled category (eg. #staff) and then recategorise it whenever you’re ready, that works pretty well. Or you can set up Shared Drafts by adding a category to your shared drafts category admin setting. That allows you to create a limited visibility topic and then publish to the correct category when ready. 
(Both of those need disable category edit notifications and disable tag edit notifications to be left default off to send out the notifications)
But I’ll attempt a repro of this bug and see what I can find out. 