Meta gem bug?

Hi, today I found out from YouTube video/streamer that beaming earthsiege diamond swapping piece of gear with it not only increase total mana by 2%, but also refunds 2% of total players mana. Blizzard fixed this about week ago on classik wotlk. I found it after 3 years playing it here, is it considered as a bug, since afterall it is game design that's been in the game whole time for over a decade? Can we use it or will warmane fix it as well, or is it well known and I have just been sleeping 3 years of playing wotlk here?