Symptom:
ISR4K configured with a boot statement pointing to the "flash:" directory will get stuck in a boot loop.

Boot statement will look similar to:
boot system flash:image.bin

When rebooting, the console will show:

Last reset cause: LocalSoft
ISR4331/K9 platform with 4194304 Kbytes of main memory

........

Device not found flash:

........

autoboot: boot failed, restarting...

Conditions:
Boot statement will explicitly point to "flash" instead of "bootflash" despite being the same directory.
This issue occurs in all version (for instance: 3.16.X to 3.17.X or 16.X, 16.X to 16.Y, 16.X to 3.X).

below boot up statements will trigger the failure

boot system flash:image-name
boot system flash flash:image-name

Workaround:
from rommon mode, manually boot up the image. for example:

rommon> boot bootflash:image-name

Another workaround requires modifying the boot system configuration as follows:

boot system bootflash:image-name.bin

Notice the semicolon was removed between the flash and image-name.bin. ROMMON will check its flash device for the image-name.bin. For the ISR4K platforms, the flash device is the bootflash:.

Further Problem Description:
below IOS boot up statements work:

boot system bootflash:image-name
boot system flash bootflash:image-name

this is fixed at ROMMON release 16.9(1r).