The reason you are not seeing the text correctly on the RGBA is that you did not define the colors correctly. The text() method's parameter fill expects an integer-valued tuple, so (0,0,0,1) would result in a nearly transparent color (0=fully transparent, 255=fully opaque). Try again with

font_color_bgra = (0,0,0,255)  # black

This should resolve your issue.

On the pure RGB image the alpha-component is not considered, thus you do not see the same error on the zebra background image.