Did you google 'eggshell skull'? Because it clearly shows that legal doctrine, across many legal systems, has said for decades that no, there is no 'presumption of regularity' (at least not in the form you're hinting at here - there is a strict other definition in some legal systems which is of no relevance here). And of course, as I said above this doesn't apply directly here because eggshell skull is about the expectation of physical frailty of a person, but similar doctrines exist for other situations.