No boot without USB<->UART converter connected

Hi. I’m having a weird issue.

I have a Zero 3W, 2GB/16GB version. I’m using the newest OS image (Debian XFCE B6).
The problem is that it doesn’t want to boot unless I connect a USB<->UART converter.

Without it, the green light just stays on forever.
But when I connect the converter, everything works fine. I get both serial console and video output.

Something seems to be blocking normal booting sequence I guess, but I’m not sure what it could be.