Before the patch it used to give 100% damage done at the end of calculations, so something that hit for 20k in the battleground would hit for 40k but now after the patch it for sure isn't the same and there wasn't any mentions of it in the patch notes.
After the patch it's definitely not the same so my guesses are:
1) It's just bugged and damage done buff falls off after a certain event for some reason while visually still being there on the buff bar
2) The 100% value got changed
3) The buff doesn't happen at the end of calculations anymore
4) Something something yada yada mixed up with that 10% mitigation change
I just don't know if it's intended or not and/or if it just got missed in patch notes.