OP, we have a Gardenia (2013) the microwave is definitely vented outside, the house plans also show a vent through the roof. I am guessing the microwave was not installed to use the vent. If you go in the attic you should be able to see the vent, same goes if you go outside you should see a rectangular vent. The microwave may have a plate that was not removed, this blocks the venting to the roof vent. Also, I have 'heard' some have a setting or switch for indoor to outdoor venting. The only way I know of checking for the plate is to remove the microwave and look at the back.
I would download the installation instructions for your microwave and see what it says about this.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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