I asked the same question of the Apple Tier 3 Support person with whom I eventually spoke. Tier 1 and 2 were sympathetic, and as surprised as I was by this behavior, and ran me through various series of convoluted setting changes, none of which worked.
[Edit: If I recall correctly, we even set it up again from scratch, to try out an idea the Tier 2 guy had. Tier 1 and 2 really were excellent to work with, despite their inability to solve the problem. Both knew the system well, weren't following scripts, and were able to think outside the box and try creative solutions. It was my first experience with Apple Support, and Gold Stars: would call again.]
Tier 3 was brusque, and acted like I was the first person who'd ever raised the topic (which, given the previous surprise, maybe I was). I explained that my dad was bedridden, and wanted to watch YouTube videos without the damn screen spinning around on him, and recall a palpable lack of sympathy. I didn't get an answer as to whether it's a bug or not, and I'm pretty sure the ticket died at Tier 3, so I doubt anyone knowledgeable will ever know about it.
In Assistive Access mode? What version are you on?
My dad passed in the spring, so it's moot for me, but I'd be glad to know that they've fixed it for other people.
[Edit: someone else in this thread said it's still bugged in 18.6.2, so if you've got a working build it'll be particularly helpful to know which it is. Maybe even for Apple - I maintain a fond delusion that folks who matter still read this board.]
Sorry, I may not have explained the problem well. Yes, in AA mode, the screen is always unlocked, regardless of any device settings you may otherwise, er, set.
