The effect of this card is: “While this creature is on the battlefield, you take double damage and this creature takes double damage”. But when my creatures attack the opponent, he takes no double damage at all and i lose a quick battle because of this stupid bug. 
I noticed an issue regarding this in the Avacyn’s Madness event. The number displayed as damage taken is incorrect but the actual damage deducted is indeed doubled. Did you double check the life totals before and after?
Yes i’ve checked it twice, but i think it starts to happen when the GC reinforced. Too bad i couldn’t capture the screenshot.
I use this card a lot, and I can say I’ve always taken double damage. You are correct that the damage displayed is incorrect.
This should just be a graphical bug, and double damage is still being applied. I checked the database, and there is already a bug report listed in there for this issue.