Probably you build boot_LS_arm64_lts_5.10.tgz with root permission.
Please delete boot_LS_arm64_lts_5.10.tgz first.
$ rm -rf build/images/boot_LS_arm64_lts_5.10*
Then rebuild bootpartition with the following command using a "non-root" user account.
$ flex-builder -i mkboot
Then install again with the following command.
./flex-installer -b bootpartition_LS_arm64_lts_5.4.tgz -r rootfs_lsdk2012_ubuntu_main_arm64.tgz -m ls1046ardb -d /dev/sdb