I know they were double xplast 2 times. I wasn’t paying attention and fed my 100 might artifact. Then I noticed that they were not doubling.
I tested with a 20 exp and it doubled for me, about 30 min ago. Remember, though, if you're feeding in an artifact, you'll get the full value of it during double exp.
It's a bit misleading yeh, I had to check few times myself when feeding old artifact of rank 84. But like others already mentioned, normally you only get 50% of the xp from the artifact but this weekend you get 100%. Or if you're lucky even more! I actually got 11k more xp from the artifact!