When creating a common/combined post for multiple platforms, when you save as draft, the draft is split into multiple drafts (one for each platform). If you want to edit the draft you have to do it for each platform rather than just one common draft as it was when you first saved it as draft.
Obviously if you’ve already split a post in order to customize for each platform you’ve selected, that would be the expected behavior. But saving a draft for a combined post for multiple platforms should be saved as a single draft, at least that’s what I would expect.