I created a box in Blender and exported that into an STL file and it seems to be failing to render the correct walls. The box looks like this:
But when I open it in Cura, it looks like this:
Is there something I'm exporting incorrectly or something else I'm missing?
This may be related to a characteristic of the numerous slicers methods of handling single thickness walls. If your model's wall is of insufficient width, it "ceases to exist" in the slicer. Ultimaker's community forum has a reference that may be of value, second post to the linked page:
This is known. It's a limitation of how Cura calculates these things: Your model outline is inset by half the line width. If anything remains, it is printed with the normal line width. If nothing remains, only the "Print Thin Walls" option may still save you. If two lines overlap a lot, then one of the two will have its extrusion rate reduced to prevent overextrusion. You'll ooze some filament there but hopefully not too much since there is a backpressure from the earlier printed line.
According to that same forum (and likely other locations on the internet), there is a setting for "print thin walls" that may solve the problem. The original poster discovered that it did not, for his model and had to increase wall thickness in the design software.
To me the resulting print pattern reminds me of a common error with blender: the model contains either no volume, not enough volume as noted by fred_dot_u or the surfaces are inverted.
Further reading to fix your model:
