Moto G5 Plus bootloops when trying to use initramfs

I have a Moto G5 Plus (XT1687, non-Amazon) running `motorola/potter/potter:7.0/NPN25.137-33/35:user/release-keys`. No matter what I do, the device bootloops until I clear the fsg-id property.

I have extracted the initramfs from both "POTTER_NPN25.137-33_cid50_subsidy-DEFAULT_regulatory-DEFAULT_CFC.xml.zip" and "POTTER_NPN25.137-35_cid50_subsidy-DEFAULT_regulatory-DEFAULT_CFC.xml.zip" by using [this perl script](https://www.enck.org/tools/split_bootimg_pl.txt) on the boot.img inside the .zip archive, and have been flashing these unmodified. However, neither of them work; I've tried 16MiB, 32MiB, or 64MiB padding. The phone gets stuck in a bootloop at the Motorola logo every time. I am sure the phone is still running a vulnerable firmware version.

Is there something I'm missing or doing wrong? Is there something I have to modify in the initramfs image so that it boots successfully? Any ideas or help would be greatly appreciated; I'd really like to be able to get initroot to work.

Output from my terminal:
```
$ head -c $((32*1024*1024)) /dev/zero > pad
$ cat pad initramfs-original.cpio.gz > initramfs-padded
$ ls -l initramfs-original.cpio.gz pad initramfs-padded
-rw-r--r-- 1 htcg htcg 1601124 Apr 9 21:43 initramfs-original.cpio.gz
-rw-r--r-- 1 htcg htcg 35155556 Apr 9 22:10 initramfs-padded
-rw-r--r-- 1 htcg htcg 33554432 Apr 9 22:09 pad
$ fastboot flash bagel initramfs-padded
(bootloader) has-slot:bagel: not found
target reported max download size of 536870912 bytes
sending 'bagel' (34331 KB)...
OKAY [ 0.951s]
writing 'bagel'...
(bootloader) Invalid partition name bagel
FAILED (remote failure)
finished. total time: 0.963s
$ fastboot continue
resuming boot...
OKAY [ 0.015s]
finished. total time: 0.015s
```
(at this point the device continuously gets stuck at the Motorola logo for a while, then reboots)