WebKit and Chromium are hard forks. The former is a fork of KHTML, and the latter is a hard fork of the former. However, in recent years I’ve only seen soft forks, and as for hard forks, I’ve only seen one with Pale Moon, which hard forked Gecko and named it Goanna due to disagreements with the direction the Mozilla Project was taking.
But why wouldn’t any organization make a hard fork, whether of WebKit, Chromium, Firefox, or another browser not based on the three mentioned above?
I’m not sure that there would be a good reason to do it to be honest. Each engine is pretty mature and has been built into its design philosophy for years - nearing decades. If you have a different idea for how a browser works you’re probably still better off starting from scratch than trying to hodge podge your vision from an existing engine that is already unwieldy and has plenty of vulnerabilities (while more and more will appear as time goes on).