Alright, here's what I could find. The answers written with regular test are based on what I could find from real developer answers, answers in italic are my personal speculation.
Keep in mind that I am just a player, not a developer. Hence, I can't guarantee that the answers are correct or up-to-date, all I can is try my best.
Q: Why are replacement premium tanks for the Chinese Type 59 and WZ-111
being created as opposed to placing them back in (or implemented on
A: Type 59 was removed from the sales, because for a premium tank, it was overpowered (it was as good as a regular tank). WZ-111 is scheduled to remain exclusive on Chinese server. Both will be replaced by a proper heavy and medium hightier vehicle (the heavy tank was named "112" and it's possible it will appear in 8.7. Nothing is known of the medium tank.