The bug

Some models get very dark shadows when in certain positions:
This happens even when the models aren't on top of each other/overlapping, as seen in the images below. The rail in the image seems to be affected by blocks that aren't even touching it.

Update:

I've noticed a pattern since reporting the bug. These shadows appear when the model is larger than the "block space." So if the model occupies more than 1 block of space, these shadows seem to appear.