macOS High Sierra 10.13 briefly flashes screen content last seen before locking the screen when awaken
Yes - the frame buffer should be scrubbed / discarded as part of the sleep / lock process no matter how brief the render. I don't think it's a huge risk and can be mitigated as you say by logging out of the screen before locking or any number of other ways like timed lockouts.

I'd just log out or enable fast user switching and switch away from the screen instead of letting the screen saver lock.

Apple publishes a page with instructions on how to report issues. If this were a huge gap, I'd consider asking you to delete the thread, but I don't think this is "stop the planet" serious unless there's a way to abuse this "feature".

https://support.apple.com/en-us/HT201220


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