NOTICE: Topic drafts are currently bugged.
If you close out of a draft with the intent to return to it later, go to your Profile --> Activity --> Drafts to reopen it. Otherwise, you risk accidentally deleting the draft.
You can also click on your avatar in the top-right, click on your Username from the drop-down menu, and then click on “Drafts” to reach the below page.
It appears that this is an issue with the latest version of Discourse and some official plugins, so I don’t have an ETA for when it’ll be resolved. I suggest sending private messages to yourself for larger drafts. Post/reply drafts still seem to work as intended.