I will give a brief definition of flat and then deflated, so we can draw a conclusion on what the prompt means. This is because together it looks like a figurative expression at first but I later so a thin line in making it literal as it is, in this case it would seem, one works directly with the other.
So here are the definitions:
Flat can be said to be level, plane. even. being level or straight or regular and without variation as e.g. in shape or texture; or being in the same plane or at the same height as something else (i.e. even with) adjective. horizontally level. Link
While deflation means to release air or gas from something or to cause to move from a higher to a lower level : reduce from a state of inflation. deflate the national economy. Link
So from the definitions above we can say that the prompt is somewhat literal because something can be flat because it was deflated, they both work hand in hand. So, flat but not deflated is a natural flatness without being deflated.
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