Transparent Artboard Fails At 360 Pixels Or Less
At 360 pixels or less in either dimension the transparency of the artboard fails and produces a white backgrounded PNG file.
361x361 > Transparent
360x361 > Fails to white
361x360 > Fails to white
Very easy to test. Create artboards and place a single text item on it. Save as PNG. Test for transparency in the manner easiest for you.