I want to say natural, fossilized mud cracks like this rock I found but yours breaks that sort of pentagonal pattern and the grooves seem less weathered. So I don’t think that’s it. But I also don’t see why a native would chose to carve a rock like that...
To clarify myself, my best natural explanation would be mud cracks but I don’t think that’s the case here. I agree your rock has patterns that differ from what you see in mud cracks. I don’t know what it is but I’d rule out mud cracks... sometimes eliminating options helps to figure out what it is.