"Do not disturb" mode activates on my iPhone without a reason

Do Not Disturb setup has been made much more flexible, but this has come at the cost of much more complication. To get it working again, do these steps ONCE:

• Tap Settings
• Tap Focus
• Tap on any Focus settings that you will never use and remove them (you can add them back later if you change your mind, and even create new, custom ones)
• Tap Do Not Disturb
• Tap People
• Tap Allow Notifications From
• If anyone is listed who should not call you when DND is on delete them; if you don’t want any calls from anyone, any time, delete all people
• Tap the dropdown next to Allow Calls From and choose Allowed People Only
• Optionally, tap on Apps, then Allow Notifications From and don’t add any apps.

All of the above is a one-time setup. After that, turn DND on or off with the Control Center Focus button. You can optionally set one or more schedules by going to:

Settings/Focus/Do Not Disturb/Set a Schedule.