Base layer not printing unless stl model rotated 90 degrees  

Base layer not printing unless stl model rotated 90 degrees

I am printing something that begins with two simple base areas. However, the first layer of one base area does not appear in the Prusa Slicer display at the zero x position, and subsequently does not print (extrude). As a result, the next layer is extruded above a non-existent base. (The other base is shown at the zero position and printing works fine.)

However, when I rotate the model by 90 degrees around the z-axis, everything works fine. All base areas are visible at the zero level in Slicer, can be sliced and exported as g-code, and printing works as expected.  Is there something I'm missing? I discovered the rotation fix by accident. If the missing base is too small, or is in a negative position then it seems that rotation would not fix the problem.  Someone on a chat used the same STL file but could not reproduce the problem.     I'm using PrusaSlicer version 2.7.2 +win64.