Dirt and debris collect on the Shimano SPD pedals. The cleat will cram the dirt in the slot and not cause the cleat to not engage all the way. They do not shed mud or any thing for that matter.
Yesterday when I was out for a ride I got small piece of a pinecone stuck in between my cleat and pedal, jamming it into that “slot” causing it to not clip in. When I stopped to look at it I had to use a small allen key to force it out.
Great news that they work for you. They do not work for me or the OP.