Compare your kernel cmdline when powered on normally to when you power on by connecting the usb cable only.
If you find a specific option to distinguish the two boot modes, please update the [https://gitlab.com/postmarketOS/pmaports/blob/master/main/postmarketos-mkinitfs/init_functions.sh#
L307- 332 initramfs script].
==== DirectFB/FBDev: No supported modes found in /etc/fb.modes and current mode not supported! ====