Comment 24 for bug 1848856

The \r is an invisible return character, commonly returned by an echo. However, here, we are executing grub_probe --target=device /boot

The code in 10_linux_zfs:
 initrd_device=$(${grub_probe} --target=device "${boot_dir}")

The results are the same as above. For the specific entry, it is the first drive in my mirror:
/dev/sdc1 (first drive as defined in my mirror)
/dev/sda1 (second drive as defined in my mirror)
/dev/nvme0n1p2 (zfs logs)
/dev/nvme0n1p4 (zfs cache)

Notice that the results of the output from grub_probe has new lines. New lines are usually \n but may be \r\n. I don't know what grub_probe is doing and didn't look to see how it does it. But the return from there would be what's got the \r (again it isn't visible).

I have nothing special or different from the stock Ubuntu /etc/default/grub other than I added zswap entries to the /etc/default/grub and those don't affect this.

The underlying cause may be that because I have a mirrored root using zfs mirroring, multiple devices show up and may cause the \r to be returned. It's just a guess.

So, to test, you'd want grub_probe to return multiple devices and the easiest way would be to just have a mirror for the rpool.