useradd doesn't seem to work correctly on a separate partition for home directories - yocto

Question

I've started recently to create a distribution for a qemux86_64 machine using dunfell LTS branch. To be able to enable the read-only-rootfs image feature and have a writeable home directory, I've added a custom wks-file:
part /boot --source bootimg-pcbios --label boot --active --align 1024
part / --source rootfs --ondisk sda --fstype=ext4 --label root --exclude-path=home/
part /home --source rootfs --rootfs-dir=${IMAGE_ROOTFS}/home --ondisk sda --label home
bootloader --timeout=0 --append="rw oprofile.timer=1 rootfstype=ext4"
With this file in place I get a separate boot- and home-directory which works great.
The next thing I wanted to add was a user within its own recipe based on the Description in meta-skeleton/recipes-skeleton/useradd/useradd-example.bb which looks like the following:
SUMMARY = "Create test users"
DESCRIPTION = "test"
LICENSE = "MIT"
inherit useradd
USERNAME="test"
USERADD_PACKAGES = "${PN}"
USERADD_PARAM_${PN} = "-u 1000 -d /home/${USERNAME} -r -P 'test' -s /bin/bash ${USERNAME};"
do_install () {
install -d -m 755 ${D}/home/${USERNAME}
# The new users and groups are created before the do_install
# step, so you are now free to make use of them:
chown -R ${USERNAME} ${D}/home/${USERNAME}
}
FILES_${PN} = "/home/${USERNAME}"
INHIBIT_PACKAGE_DEBUG_SPLIT = "1"
...but unfortunately it doesn't seem to work correctly. The /home/test directory is created, but with the uid and gid from the user triggering the image build. Same issue with the /home/root directory.
The issue disappears as soon as I remove the home partition from the wks file, but I wouldn't expect that a separate partition is unusual, especially if you need a location for persistent data with the read-only-rootfs feature enabled.
That said, I'm confident that I've messed up something, but unfortunately I can't find the missing piece... Would be great if someone could help me resolving this issue.