Trying to use a Rule to Update a Checkbox field to “unchecked” - the obvious solution does not work:
You can make it “checked”, but you cannot make it “unchecked” 
Workaround is to use a Formula = “False”
There’s an option under the three dots to clear the value
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So it’s a feature, not a bug? 
Seriously, why not just let the obvious, simple thing work?