The more recent builds of Sigil are the first to be built with OSX 10.12 as the minimum which is what triggers this nonsense. That is why the Sigil 0.9.9 and 0.9.10 do not see the same issue.
I did find this Qt bug but of course nothing has been done about it ....
https://bugreports.qt.io/browse/QTBUG-61707
Sometimes I really think Qt is not being properly managed (to say it as politely as I can).