MacBook Pro (16-inch, 2021) Shuts Down on Boot Without External Monitor

I'm having an issue with my MacBook Pro (16-inch, 2021, M1 Pro). The laptop shuts down either during the boot process or shortly after logging in only when an external monitor is not connected.

When I connect an external display (via USB-C/HDMI), the Mac boots and runs normally. But if I try to use it standalone without any external monitor, it either turns off at the Apple logo or shuts down within a minute after logging in.

I've tried resetting NVRAM and SMC (as much as is possible on M1), and booting into Safe Mode, but the problem persists.

Has anyone else experienced this issue or found a fix?

Thanks!