[Cura not rendering correctly box with cut out region on bottom layer]

Hi.  I'm new to 3D printing.   I'm creating a custom box to hold my modem and routers.   The bottom layer of the box has a cut out region.

When I slice it and view layers in preview mode, for some reason Cura is rendering the bottom layer of the box as if there was no cut out region.

Using Cura 4.8.  No raft or brim.  Happens with our without support.

Am I doing something wrong?

See screenshots below, as well as the attached STL file.

Thanks for any help!

 

 

 

 

 

router_box.stl