I get the same problem using Matlab 2012b. I've been finding a number of weird behaviors with Mapping Toolbox plotting functions lately. The first thing I always try as a workaround is to manually perform coordinate transformations, then plot using standard plotting functions. To do this,
Initialize a map,
Transform coordinates, then
plot using standard (non-mapping toolbox) plotting functions. Here's an example:
% Using these coordinates for your bounding box:
lat = [-60 -60 60 60 -60];
lon = [-140 140 140 -140 -140];
% Initialize a map:
worldmap('world')
% Perform coordinate transformation:
[x,y] = mfwdtran(lat,lon);
% Plot using line instead of linem:
line(x,y,'color','m')
However, the bounding box does not follow the curves of the projection, so you might want to densify the lines before doing coordinate transformation: