And your XUbuntu uses a library like uClibc or musl as the underlying C function library for all binaries?
Working with uClibc, musl and busybox etc. requires some deviation from the full desktop distro behaviours.
You might familiarize yourself with the uClibc and musl internals...
Openwrt behaviour with musl was kept in line with the old uClibc approach.
See e.g. proposal in 2014: https://lists.openwrt.org/pipermail/openwrt-devel/2014-November/029296.html
bug report in 2015: https://dev.openwrt.org/ticket/19901
leading into commit in 2015: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=7b4d039e00b0e29f416c21a022ce2a6db23a4c30;hp=23633249c80c8ae886569058349e345d3a905bbc
But I haven't looked into reasons why there is an empty symlink for /etc/localtime. Likely it has some compatiblity history.
EDIT:
Ok, this is the history. support for using the zoneinfo package: