I am experiencing a similar problem. I started using Sleep Tracking with my new iPhone 16 Pro and Apple Watch 9 after reading about the Sleep Tracking improvements in iOS 18/Watch OS11 (which the watch has been upgraded to). Every night my phone and watch go into Sleep Focus at the scheduled time, yet I receive Stand reminders (i.e. via the watch's haptics system and a sound - the same alerts I get during the day when I appreciate the reminders), two or three times a night. I also received one notification of a credit card charge having gone through which probably came from a third-party banking app.
I have had two phone sessions with Apple Support to try to address this problem. Both support advisers agreed that this is not expected behavior, and have confirmed that I have Sleep focus set up correctly. But other than stumbling around in the phone's and watch's settings, they haven't seemed to know what the source of the problem is or how to fix it.
I am literally losing sleep over this problem, so I would really appreciate it if any users have concrete suggestions.