Applying multi-block boundaries can leave uninitialized boundary points. This happens when multi-block boundaries are used in combination with another boundary condition that looks at the interior of the grid, such as e.g. a symmetry condition or radiative / extrapolating boundaries.
There is another bug currently in McLachlan that applies its "scalar" boundary conditions to too few grid points (it uses "BoundaryNoSync" instead of "Boundary"), which means there are uninitialized boundary points for
initial_boundary_condition = "scalar" as well in multi-block systems.