Here is a 16-bit grayscale image:

It clearly contains a gradient of grays.

If I open it with Pillow, convert it to 8-bit grayscale, and save it, like so...

>>> from PIL import Image
>>> test_img = Image.open('test.png')
>>> test_img.mode
'I'
>>> test_img.convert('L').save('out.png')

... then I get this, which is mostly completely white:

This conversion should work; according to http://pillow.readthedocs.io/en/4.2.x/handbook/tutorial.html#converting-between-modes

The library supports transformations between each supported mode and the “L” and “RGB” modes.

and according to http://pillow.readthedocs.io/en/latest/handbook/concepts.html#modes, I is a supported mode.

Notably, I don't see this same problem if I start by loading an 8-bit RGB image from a JPG and then do .convert('I').convert('L'), so it's not simply the case that I->L conversion is broken in general. I'm not what specifically leads to the breakage in this case.