Zhongli being made of Gold might be the reason to him not being affected by Erosion. he says that even himself can't escape Erosion and yet he still remembers EVERY contract he signed, EVERY person that he comes across and EVERY Event that happened from thousands of years ago.

Its been hinted many times that Zhongli's memory is special, so much that I still believe that he remembers Rukkhadevata because his Character Story 5 doesn't consider Nahida as one of the Origina 7 Nahida had replaced Rukkha's existence after the Sumeru Archon Quest