Because when compiling, you are using the KivyMD library from the PyPi and not from the master branch.
This is a bug in the library. Do not use the value 0, 0, 0, 1. If you need a black icon color, you can set the value to 0, 0, 0, 0.9:
MDFloatingActionButton:
text_color: 0, 0, 0, .9
md_bg_color: 1, 1, 1, 1
pos_hint: {'center_x': 0.8, 'y': 0.5}
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