how to manual?
After recompile Linux with the following command.
$ flex-builder -c linux
Please execute the following command to regenerate bootpartition tarball boot_LS_arm64_lts_5.10.tgz with the latest Kernel image.
$ flex-builder -i mkbootpartition -a arm64
Most probably hostname is changed in booting userspace ( /etc/inittab, /etc/init.d/Sxx scripts ...)
If I were you, my first check would be grep for 'hostname' command in generated rootfs tree
The hostname in kernel has lowest priority, in buildroot is default '(none)'