Clip groups snap to grid by their clipped contents and not the mask
Snap to Grid option doesn't work properly for Clipping Masks.
Instead of snapping the boundaries of the Clipping Mask to the grid, Illustrator snaps to the grid the bounding box of the masked object hidden under the mask.
Here is the demo.
Oh, yes, a dozen times yes. I am so tired of this!
There is a workaround — to grab the clipping group specifically by the clipping path, but it’s a tedious one...