Something bizarre seems to be happening where applying the X access mirror modifier mirrors on the Z access and the Z access modifier flips it over the X.
You can see in the photo below that X is selected to mirror, but it's not mirroring on the X. It's flipping on the Z instead.
I tried doing these cursor fixing steps, although it seems like it might be a different problem, since it don't work.
What went wrong? How do I fix it? This is 2.8 Beta.