Our accessibility reviewer said:

'Unticked: alt = not completed; title = mark as complete.
Ticked: alt = completed; title = mark as not complete.

When both attributes are present screenreaders under default settings will read
the alt but not the title. This means that these users will not know the
function of the button. When alt is not present screenreaders will read the
title.'

We discussed various options but agreed to change it so that the alt text also includes a version of the wording from the title. This is a language string change only.