Unfortunately, since there’s no reliable way to detect whether you’re using an external keyboard, it’s intentional to not focus when switching from preview to edit mode, because having the on-screen keyboard pop out unintentionally is really frustrating.
However, the good news is that we’ve added a new command to focus on the editor as of v0.1.1, which can be assigned to a hotkey of your choice. This should alleviate the problem of having to tap the editor to get focus back.