Direct vs Bowden Extrusion when printing in temperature fluctuating environment
I don't think your temperature based explanation makes sense. This looks to me just like a first layer smashed against the bed, possibly too much due to nozzle being too low, followed by underextrusion when there's actually the right amount of space to extrude into. If you have fewer than 3 top layers or less than 0.6 mm of top layer thickness, you will almost certainly have an underextruded top like this just from the first top layer sagging into the space between the infill lines. Since the walls look okay, this is my best guess; if the walls looked bad too I would suspect poorly tuned filament diameter/E-steps-per-mm. Also check that your infill is not too sparse; more than 4 mm or so between infill lines gives a lot of room for top layers to sag into and may increase the number of top layers you need.
I see you added in the comments that your layer height is 0.1 mm. At such thin layers, you probably need at least 5 or 6 top layers; the first 2-3 are expected to sag and not fill the space.