Are you sure it was Pandaria last week? The Black Empire invasion is in Pandaria this week, so if true then it means the invasions aren’t rotating properly.
Uldum not containing the entrance right now is working as intended, however, as Black Empire is not there.
Indeed, the week’s Black Empire assault is in the Vale of Eternal Blossoms, not Uldum.
I just took a quick run through Heroic Nyalotha just to confirm. The portal is in the floating ziggurat next to Whitepetal Lake, as expected.
FWIW, if I don’t find the Black Empire assault in the first zone I check, I always check the other, because I won’t waste brain cells trying to remember the rotation.
I had MoP Vale set to the prior time so it did not show the assault on my map at all, so when I saw it Uldum I totally forget there are assaults in both zones and the entrance swaps places.
Went back to MoP, talked to the time person and got the raid done woohoo