Can't repack Google Pixel 3 XL boot.img

[zhuowei]

To reproduce:

• grab boot.img from Pixel 3XL December factory image (crosshatch-pq1a.181205.006-factory-96b23504.zip)
• run:

bbootimg -x boot.img
bbootimg -u boot.img zImage

• try to boot the resulting image.

Expected: repacked image boots
Actual: fastboot hangs during boot:

$ fastboot boot boot.img 
Downloading 'boot.img'                             OKAY [  1.495s]
booting                                            

Reconnecting to fastboot and dumping bootloader log with fastboot oem dmesg shows that the bootloader can't detect the kernel compression type:

(bootloader) AcceptCmd:download:04000000
(bootloader) Downloading 67108864 bytes
(bootloader) AcceptCmd:boot
(bootloader) avb_slot_verify.c:524: DEBUG: Loading vbmeta struct from pa
(bootloader) artition 'vbmeta_b'.
(bootloader) avb_slot_verify.c:524: DEBUG: Loading vbmeta struct from pa
(bootloader) artition 'system_b'.
(bootloader) avb_slot_verify.c:286: DEBUG: dtbo_b: Loading entire partit
(bootloader) tion.
(bootloader) AvbLoadAndVerifyBootImages: boot state orange
(bootloader) kernel compression not supported
(bootloader) Error calling BootPrepareAsync Unsupported
(bootloader) Fastboot: Error executing command boot

This device uses the new v1 boot image header along with a switch to LZ4 compressed kernel; would that cause the issue?