Using the normal-inaccessible block collision bounds debug feature, it can be seen that the collision for north and south facing grindstones has many more components than their east and west counterparts. This applies when mounted to the wall, ceiling, or floor. East & west grindstones' collision has 5 parts, one for the wheel, and two for each wooden support, however north & south grindstones are much more convoluted. This can also be demonstrated by flying into the block in spectator mode partially, then going into survival mode. If you are lined up correctly (z+0.26 for a wall-mounted north-facing grindstone) , you will be unable to walk through certain parts of the collision that you would be able to walk through in an east/west grindstone (south in the given example)
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@Galaxy_2Alex I am using a mod to make them visible, but being stopped by sections that look like they should be one piece happens in vanilla as well, the mod just shows why it happens.
What exactly are you using to make block collision boxes visible? Is it a mod?