S8 won't shutdown and won't go into recovery mode

I can boot my Samsung S8 to Safe Mode, but nothing else.

• VOLUME UP + BIXBY + POWER (release all at Samsung screen) => boot in to safe mode
• From Safe Mode: RESTART => phone powers down but enters Android Warning screen; buttons are unresponsive
• From Safe Mode: POWER OFF => phone does not fully power off, but instead goes to Android Warning screen; buttons are unresponsive
• From Android Warning screen: VOLUME UP + BIXBY + POWER (hold through Samsung startup, waiting for Android logo) => loop of restarting to Samsung screen, then restart

The only thing I can do is get to safe mode. I tried to let the battery completely drain to power off, but once it’s dead and gets plugged back in, it goes back to the Warning screen loop. Help!