https://clips.twitch.tv/SuccessfulCoyDoveBCouch
The gem was socketed into the chest piece before it was equipped and wasn't giving the correct amount of life until the gem was unsocketed, once re-equipped it gave the correct amount of life (without the gem), and the correct amount of life (with the gem) once the gem was resocketed into the item.

e/ This was supposed to be in the bug report section, my mistake!