Problem with fields at physical boundaries in 1D

[c-white]

In 1D SR shock tubes (possibly also Newtonian; have not checked), something is going wrong with the B-fields at the boundary. Using outflowing boundary conditions (so the boundary area should not change at all for many timesteps), face-centered Bz values are wrong after the first half step (at least with MUB shock tube 2, though all MHD shock tubes are failing regression tests for presumably similar reasons). The error is in the two ghost cells and the first active cell at this point.

This is all done in one mesh block on one core.

[tomidakn]

Xuening reported a problem about the magnetic field boundary condition, so I guess this is related.