Hey Sinrai. What you are seeing is the discount currently existing on the live servers. There is an achievement on the live servers for equipping two Legendary implants at iLevel 330 (it may be a different iLevel, but if I remember correctly it is 330). This achievement unlocks access to lower cost for future Legendary implant purchases/upgrades - which is 2,500 Tech fragments. So currently on live, the cost is 2,500 Tech frags (with the 330 achievement reduction).
What I’m assuming happened in your case… Since achievements don’t copy to PTS and you immediately went to the Legendary implant vendor to check prices prior to equipping two Legendary implants of iLevel 330 or above, you hadn’t yet unlocked the 330 achievement and were being shown the full 6,000 Tech frag original cost. Once you equipped your 334 Legendary implants you were given the 330 achievement and granted the cost reduction to 2,500 Tech frags that currently exists on live. So I would say what you’re seeing is expected based on what is currently on the live servers.
I believe the issue is that no further cost reduction occurs once you are granted the iLevel 340 achievement (cost still remains 2,500 Tech frags).