What some people aren't taking into account is that the healing done by spells such as Forgemaster's Flame is -not- nerfed by the trait. Yes, Healing word/Bastion of health are, but FF isn't - and that may be where the balance comes in to play. I think a 25% nerf to self-heals is sufficient, but the present 75% is…
It's a 75% reduction, despite saying it's only 50%. Your math is correct - and yes, it's probably a bug. Either way, the trait should be "Causes your heals to do 50% more when cast on others", not "Causes your heals to do -50% (should say -75%) when used on yourself" IMHO.