verify fails usb single zfs with cache

[Kodey]

Hi,

I'm trying to create a cheap 2nd backup and experimenting with different filesystems.
I creates a zfs with a new usb3.0 14tb wd Elements (passed badblocks test).

zpool create -o ashift=12 -o feature@encryption=enabled \
             -O exec=off -O compression=off -O dedup=off \
             -O xattr=sa -O atime=on -O relatime=on \
             -O encryption=aes-256-gcm -O keyformat=passphrase \
             -O keylocation=prompt -m /mnt/datastore/USBpool \
             USBpool /dev/disk/by-id/usb-WD_Elements_25A3-0\:0 \
             cache /dev/disk/by-id/nvme-nvme.64G-part6

Backup of 4.2TB lxc takes about 4.21h but verify take ~21h and fails.

Backing up the same lxc to a xfs on an identical disk takes a bit longer but verify isn't so slow and doesn't fail.

Any ideas what's up?