No, it seems that trimap_alpha has 3 channels. I’ve double-checked now, the alpha maps are P images and the color images are RGB.
<PIL.PngImagePlugin.PngImageFile image mode=P size=800x497 at 0x22937E158D0>
<PIL.PngImagePlugin.PngImageFile image mode=RGB size=800x497 at 0x22937EBD978>

How should I process the alpha maps?