Why does changing Database Table Prefixes result in loss of content?

You should have not searched the field and replaced all the references to wp_ with anything.
For example : the value of the entry with meta_key wp_page_for_privacy_policy will be change to wp_addtionnal_prefix page_for_privacy_policy.
Hence there could be a lot of misfitted values in your table.

A better way would have been to :

  • export the DB
  • import the DB as a new DB
  • rename all tables with the new prefixes
  • export it again
  • import it in you old DB
  • rename prefix in wp_config.php
  • check if everything is allright