Why do meshes break batches on certain positions

I have a simple setup of 3 meshes (wheels) with less than 900 mesh attributes and one (cylinder) with more. The following screenshot shows them in top down orthographic view, aligned in front of the camera. the 3 wheels get batched in Depth, Shadow and Forward rendering pass, while the cylinder is rendered alone.

Moving the cylinder along the z-axis just swaps out both jobs. So far so good.

Now comes the weird part. while keeping the cylinder closer to the camera and moving one of the wheels closer to the camera too, at a certain point, the batch of the wheels break on the depth pass, round about there:

It doesn't even matter, if I move the wheel back into z position OR the cylinder, both get batching back to work again. I also see a 1 frame black screen while crossing the point where batching breaks or works. I have even tried changing LOD bias, but that did not do anything either.

So what's the reason for breaking the batch and in doing that depend on a certain position or distance from the camera?