Apple’s support docs refer to a per-text Correct Spelling Automatically menu option (which is available in TextEdit etc)…
It is also in the human-interface guidelines…
Have I misunderstood, or is this the per-text Correct Spelling Automatically option you mention in the quote above? If Scrivener included that option on the edit menu, would the auto-capitalisation be more likely to work, and allow users to have discrete settings for each project?