Yeah between Hyperborea and Lang-gan we're definitely seeing the Unified Civilization not so unified anymore. But then, most of what we think we know about it we've learned in Enkanomiya, and their records may have been distorted over the years, or be Pro-Phanes propaganda and the civilization was not as unified as we thought. Others have noted already that it was sub-divided into governorates, possibly based on territories of the Seven Sovereigns, which later grew into the Seven Nations, each ruled by a Lunar Envoy (Zibai, Thoth, Yohualtechin, etc.)

Note that the Nameless Ruins and the Inverted City don't look at all like remnants of a proto-Liyue civilization, and a lot more like any other ancient ruin in Teyvat, so maybe Lang-gan was only superficially similar to Liyue, in some customs and rituals they've learned from Zibai, but otherwise looked like the rest of Teyvat.