The code

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\starttext

Euclid \emph{Elem}.\ 3.20 states that if \m{BC} is  joined to the center
\m{O} of the circle, then \m{\angle BOC = 2\cdot\angle A},  and \m{\angle
BOC} is equal to  \m{BC\kern-1em\raise1.8ex\hbox{\frown}}.%
%
\footnote{Euclid \emph{Elem}.\ 3.20 states that if \m{BC} is  joined to the
center \m{O} of the circle, then \m{\angle BOC = 2\cdot\angle A},  and
\m{\angle BOC} is equal to \m{BC\kern-1em\raise1.8ex\hbox{\frown}}.}

\stoptext

processes as it should using earlier versions (e.g., ConTeXt  ver:
2017.03.26)  but goes awry using ConTeXt  ver: 2017.05.06.

So far as I can tell, the problem comes with the specification of the
fonts. If one omits the stuff before “\starttext”, the footnote looks as it
should and everyone but the publisher would be happy.

Alan