Moto G4 Rooted stuck on TWRP recovery loop
I don't know if it's still a problem, but on my Moto G5 I have a similar problem, but booting into the bootloader and selecting start seems like the only thing that is able to boot the device.

Start will automatically be selected. If not, use the volume buttons to select it. After selecting it press the power button

No answer yet, but it could be useful in the future.
TWRP saying no OS is installed