My phone is a s9+. My phone was opening up in safe mode nonstop today. I tried turning it off via drop-down widget, it reset the phone, but still opened back up in safe mode. I then tried simply restarting the device and it kept opening back up in safe mode. I then restarted the phone and opened up the maintenance boot mode. This is where I am currently at. I've been stuck on this screen for a while now. I can scroll with volume up or down. I cant select anything with the bixby button. and holding the power button only makes the phone go back and then reopen back up to the maintenance boot mode. It won't turn off or anything. I'm simply stuck.
If you can't shut it down to reboot, let the battery die, charge it up a little and reboot. I don't now if this will work but it's worth a try. Sort of like a battery pull on older phones that had removable batteries.
We are sorry you are having trouble with your device. Have you been able to get out of the maintenance menu? Can you get your device out of Safe Mode?